UPS 2

Eng nginee ineerin ring g Enc Encycl yclop ope edia Saudi Sa udi A ramco DeskTop Standards

Batteries Batteries An d B atte attery ry Chargers

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : El Electrical File Reference: EEX21102

For additional information on this subject, contact W.A. Roussel on 874-1320

Engineering Encyclopedia

Electrical Batteries and Battery Chargers

C O NT E NT S

P AG E

DETERMINING TYPES OF BATTERIES FOR USE IN TYPICAL SAUDI ARAMCO APPLICATIONS APPLICATIONS ................................................................................... 1 DETERMINING BATTERY SIZE FOR USE IN TYPICAL SAUDI ARAMCO APPLICATIONS APPLICATIONS .............................................................................................. 34 DETERMINING BATTERY CHARGER SIZE FOR USE IN TYPICAL SAUDI ARAMCO APPLICATIONS APPLICATIONS ................................................................................. 57 WORK AID 1: PROCEDURE PROCEDURE AND TECHNICAL TECHNICAL AND ECONOMIC REQUIREMENTS FROM SADP-P-103 AND ESTABLISHED ENGINEERING PRACTICES FOR DETERMINING THE TYPE OF BATTERY FOR USE IN TYPICAL SAUDI ARAMCO ARAMCO APPLICATIONS...................................... APPLICATIONS...................................... 71 WORK AID 2: PROCEDURE PROCEDURE AND TECHNICAL TECHNICAL REQUIREMENTS REQUIREMENTS FROM SADP-P-103 AND ESTABLISHED ENGINEERING PRACTICES FOR DETERMINING THE SIZE OF BATTERY FOR USE IN TYPICAL SAUDI ARAMCO APPLICATIONS APPLICATIONS .................................................................... 73 WORK AID 3: PROCEDURE, PROCEDURE, TECHNICAL TECHNICAL REQUIREMENTS, REQUIREMENTS, AND AND FORMULAS FROM SADP-P-103 AND ESTABLISHED ENGINEERING PRACTICES FOR DETERMINING THE SIZE OF A BATTERY CHARGER FOR USE IN TYPICAL SAUDI ARAMCO ARAMCO APPLICATIONS...................................... APPLICATIONS...................................... 85 GLOSSARY ........................................................................................................................ 88



C O NT E NT S

P AG E

DETERMINING TYPES OF BATTERIES FOR USE IN TYPICAL SAUDI ARAMCO APPLICATIONS APPLICATIONS ................................................................................... 1 DETERMINING BATTERY SIZE FOR USE IN TYPICAL SAUDI ARAMCO APPLICATIONS APPLICATIONS .............................................................................................. 34 DETERMINING BATTERY CHARGER SIZE FOR USE IN TYPICAL SAUDI ARAMCO APPLICATIONS APPLICATIONS ................................................................................. 57 WORK AID 1: PROCEDURE PROCEDURE AND TECHNICAL TECHNICAL AND ECONOMIC REQUIREMENTS FROM SADP-P-103 AND ESTABLISHED ENGINEERING PRACTICES FOR DETERMINING THE TYPE OF BATTERY FOR USE IN TYPICAL SAUDI ARAMCO ARAMCO APPLICATIONS...................................... APPLICATIONS...................................... 71 WORK AID 2: PROCEDURE PROCEDURE AND TECHNICAL TECHNICAL REQUIREMENTS REQUIREMENTS FROM SADP-P-103 AND ESTABLISHED ENGINEERING PRACTICES FOR DETERMINING THE SIZE OF BATTERY FOR USE IN TYPICAL SAUDI ARAMCO APPLICATIONS APPLICATIONS .................................................................... 73 WORK AID 3: PROCEDURE, PROCEDURE, TECHNICAL TECHNICAL REQUIREMENTS, REQUIREMENTS, AND AND FORMULAS FROM SADP-P-103 AND ESTABLISHED ENGINEERING PRACTICES FOR DETERMINING THE SIZE OF A BATTERY CHARGER FOR USE IN TYPICAL SAUDI ARAMCO ARAMCO APPLICATIONS...................................... APPLICATIONS...................................... 85 GLOSSARY ........................................................................................................................ 88



DETERMINING TYPES OF BATTERIES FOR USE IN TYPICAL SAUDI ARAMCO APPLICATIONS The process of determining the type of battery for use in a given application requires requires an evaluation of factors such as the following: • • • • • • • •

The The type type and the dur duratio ation n of the the conne onnect cted ed loa load. The The an antici ticipa pate ted d fr freque equen ncy and dep depth of dis disch chaarge. rge. The The amb ambiient ent te temper mperaature ture at th the in insta stalla llatio tion sit sitee. The pl planned li life of the installation. The frequency of of ma maintenance. Space limitations. Life cycle cost. Seismic requirements.

The two general types of storage batteries that can be used in Saudi Aramco installations are lead-acid batteries and nickel-cadmium batteries. Each of these general types offers certain advantages and disadvantages disadvantages in regard to the previously previously listed factors. factors. In addition, a number of individual designs exist within within each general type. These individual designs designs offer further advantages and disadvantages in regard to the previously listed factors. Because no single battery design can provide the optimum performance that is associated with each of the previously listed factors, the factors must be weighted as to their importance in each given installation. installation. The actual battery type type is then determined based based on the weighted factors to provide the best available compromise between desirable characteristics and undesirable characteristics. This section of the Module will provide information on the following topics that are pertinent to determining the type of battery for use in typical Saudi Aramco applications: • • • •

Lead-Acid Batteries Nickel-Cadmium Ba Batteries Operational Characteristics Battery Applications



Lead-Acid Batteries The lead-acid battery is a group of electrochemical cells that are connected in series to generate a nominal dc voltage that would supply power to a suitably connected electrical load. The number of cells that are connected in series determines the nominal voltage rating of the battery. The amount of active material that is contained in an individual cell is the factor that determines the discharge capacity rating of the cell; the rated capacity of the individual cells is the rated capacity of the entire battery. Connecting the individual cells in series does not increase the capacity rating of the battery. The electrochemical couple that is used to form lead-acid cells is configured through placement of a lead active material in a dilute sulfuric acid electrolyte. This electrochemical couple produces a nominal cell voltage of approximately two volts. All lead-acid cells are constructed through use of the following basic components that are shown in Figure 1:

• • • •

Element Cell jar Cell cover Electrolyte



Basic Components of a Lead-Acid Cell Figure 1



Figure 2 shows an exploded view of the battery element. The battery element is the key component of the cell, and it consists of an assembly of positive and negative plates that are insulated from each other through use of separators. The interaction that occurs between the battery element and the electrolyte determines the cell's performance characteristics. Each of the plates consists of a rigid lead alloy that provides physical support for the relatively porous active materials. The positive and the negative plates are sandwiched together in an alternating pattern (e.g., negative-positive-negative-positive-negative) with a negative plate at each end of the assembly. Each positive plate is separated from its neighboring negative plate by an insulating material. The insulating material typically is a thin sheet of microporous rubber or plastic that is ribbed on the side that faces the positive plate.

Battery Element Figure 2



The cell jar, which was shown in Figure 1, is usually made of a transparent impact-resistant plastic material. The cell jar must be large enough to enclose the element and still provide sufficient reservoir space above and below the element. The upper reservoir space is needed to allow for the gradual lowering of the electrolyte level that occurs during normal operation of the battery. The lower reservoir space serves as a collection basin for the sediment that is shed from the plates during the expected life of the battery. This sediment must be kept away from the element to prevent short-circuiting of the positive and the negative plates. After the element has been lowered into the empty jar, the cell cover is placed over the top of the jar and is sealed. The terminal posts that are connected to the element protrude through holes that are in the top of the cell cover. The cell cover also is fitted with a flame arrestor vent that allows gases to escape from inside of the cell but that prevents entry of sparks or flames. The electrolyte that is used in lead-acid batteries is a dilute solution of sulfuric acid and water. The ratio of acid weight to water is measured as specific gravity. Pure water has a specific gravity of 1.000. A typical nominal specific gravity for a lead-acid storage battery is 1.215 at 25 oC (77oF). The specific gravity of the electrolyte gradually drops as a cell is discharged. When the battery charger recharges the cell, the specific gravity gradually rises back to the nominal value. If a battery is to be operated at temperatures that exceed 29 oC (85oF) for more than 30 days per year, a tropical (low) specific gravity electrolyte could be used to increase the life of the battery. A medium or a high specific gravity electrolyte is also available for special applications such as UPS systems that, in some cases, will reduce the overall battery size that is required. As previously stated, all lead-acid batteries contain the same basic components. The major difference between the various types of lead-acid batteries is the design of the positive plates that are used in the battery element. The remainder of this section provides information on the following specific types of lead-acid batteries:

• • • •

Plante Lead calcium Lead antimony Sealed



Plante

The Plante lead-acid battery is named after Raymond Gaston Plante, who was the inventor of the first practical lead-acid storage battery. The original Plante cell was constructed of two long strips of lead foil and intermediate layers of coarse cloth that were spirally wound and immersed in a 10% solution of sulfuric acid. Because the amount of stored energy of the early Plante cells depended on the corrosion of one lead foil to form lead dioxide, which is the active material of the positive plate, the early Plante cells had little capacity. However, the capacity of the early Plante cells did increase after repeated cycling because the cycling resulted in corrosion of the substrate foil. Such corrosion created more active material and an increased surface area. Today, the name "Plante" refers to all lead-acid storage batteries in which the active material of the positive plates is electro-chemically developed from pure metallic lead. Modern Plante lead-acid batteries are available with two types of positive plate constructions: traditional and Manchester. Both types of plates are shown in Figure 3.

Positive Plates Used in Plante Lead-Acid Batteries



Figure 3



The traditional positive plate starts off as a blank (slab) of pure metallic lead. Grooves are then cut into the surface of the lead blank through use of a combing process. The grooves increase the surface area of the plate, which increases the capacity of the cell. The Manchester plate is the most common type of plate that is used in Plante lead-acid batteries. The Manchester plate is constructed of a heavy antimony alloy grid that is cast with circular holes. Heavy corrugated strips of high purity lead are then rolled into spiral buttons or "rosettes" that are forced into the holes that are cast in the grid. Each of the lead buttons exposes approximately five times more surface area to the electrolyte than a comparable area on a pasted plate battery. The lead buttons also help to prolong the life of the cell by providing a reserve supply of unformed lead that is gradually converted to active material during operation of the cell. The negative plates of Plante lead-acid batteries are pasted or "flat" plates with a heavy alloy grid. The construction of these plates will be discussed in more detail in the next section of this Module. The following are the major advantages and disadvantages of Plante lead-acid batteries:

Advantages • Long service life (20 to 25 years) • Smallest amount of positive plate growth of all lead-acid cells • Very high reliability

Disadvantages • High cost • Poor energy density and poor power density •Moderate self-discharge rates (3%/month)

Plante lead-acid batteries have the highest capital cost of all types of lead-acid batteries. The cost of Plante lead-acid cells is approximately 3 to 3-1/2 times that of a comparable lead antimony cell and approximately 2-1/2 to 3 times that of a comparable lead calcium cell. However, in some applications, the high initial capital cost may be justified by the long (25 year) service life. The cyclic performance and cycle life of Plante lead-acid batteries, as is the case with all stationary lead-acid storage batteries, can only be discussed in relative terms. Stationary leadacid storage batteries are designed to provide a relatively long calendar service life (more than ten years) when they are operated under float charge conditions rather than a Lead-Acid Batteries (Cont' d) long cycle life. Stationary lead-acid storage batteries are normally not rated or guaranteed to deliver a specific number of equivalent full charge cycles.



When any stationary lead-acid storage battery is subjected to deep and frequent discharges, its calendar service life will be reduced; however, because of the number of variables that are involved, the specific amount of calendar service life reduction cannot easily be quantified. For this reason, the cyclic performance of one type of lead-acid storage battery is generally only discussed in terms of comparison with another type of stationary lead-acid storage battery. The cyclic performance (e.g., the ability to withstand frequent and/or deep discharges) of Plante lead-acid batteries is far superior to the cyclic performance of lead calcium batteries and is approximately equivalent to the cyclic performance of lead antimony batteries. Plante lead-acid batteries are capable of withstanding a moderate amount of cycling with a minimum loss of calendar service life. Lead Calcium

The positive and the negative plates of lead calcium batteries are pasted plates. Pasted plates are constructed by forcing a thick slurry of active material (e.g., a combination of lead oxides and sulfuric acid) into an open lattice grid. A typical pasted plate grid is shown in Figure 4. The open lattice grid is cast from an alloy of pure lead and calcium. The addition of the calcium alloying agent is necessary to increase the mechanical strength of the plate.



Typical Pasted Plate Grid Figure 4 After the active material is "pasted" on the open lattice grid, the plates are dried and formed (activated) by an electrochemical process. The completed plate is porous so that the sulfuric acid electrolyte can circulate through the active material. The porous construction greatly increases the surface area of active material that is in contact with the electrolyte, which increases the capacity of a given sized cell.



The following are the major advantages and disadvantages of lead calcium batteries:

Advantages • Medium service life (12-15 years). • Better energy density and power

• • •

Disadvantages • Subject to excessive positive plate grid growth. • Not suitable for deep or density than Plante batteries.frequent discharges

Low self-discharge rates (1%/month). Low water consumption. Medium cost.

The cost of lead calcium batteries is more than the cost of lead antimony batteries but is considerably less than the cost of Plante batteries. On a cursory examination, this medium range cost, when coupled with the other advantages of lead calcium batteries, make this battery appear to be more attractive for most applications than the Plante battery. However, a close look at the disadvantages of the lead calcium battery shows that this battery is only more attractive in non-cycling applications. The cyclic performance of lead calcium batteries is extremely poor. Of the three major types of stationary lead acid storage batteries (Plante, lead calcium, and lead antimony), the lead calcium battery suffers the greatest loss of calendar service life when it is subjected to cyclic service. This battery should only be used in float charge, shallow cycle applications. Lead Antimony

The positive and the negative plates of lead antimony batteries are also pasted plates. The only real difference between the construction of lead antimony batteries and the construction of lead calcium batteries is the addition of an antimony alloying material to the grid rather than a calcium alloying material. The antimony alloying material, which is similar to the calcium alloying material, is added to increase the mechanical strength of the plate. The amount of antimony that is used in the grid varies from about 1.5% to 12% antimony by weight. The percentage of antimony that is used affects the characteristics of the battery. High antimony content provides greater grid strength; however, it also results in higher float current requirements, an increase in water usage, and an increase in the frequency of equalizing chargers towards the end of the battery's service life. Low antimony content provides more desirable operating characteristics but at the sacrifice of grid strength.



If the antimony content is less than 4%, small amounts of other elements such as selenium must be added to maintain sufficient grid strength. Saudi Aramco does not allow the use of lead antimony batteries that contain more than 3% antimony. The following are the major advantages and disadvantages of lead antimony batteries: Advantages • Low cost. • Better energy density and power density than Plante batteries. • Can be used in cycling service.

Disadvantages • High self-discharge rates (7%/month). • Short service life (10-12 years). • High water consumption.

Lead antimony batteries have the least capital cost of all lead-acid batteries. The low capital cost makes this battery an attractive choice for cycling service applications in which the high water usage is acceptable. The cyclic performance of lead antimony batteries is approximately equivalent to the cyclic performance of Plante batteries. Lead antimony batteries are capable of withstanding a moderate amount of cycling with a minimum loss of calendar service life. Sealed

Two different versions of sealed lead-acid batteries are available: the gelled electrolyte version and the absorbed glass mat (AGM) version. The basic technology of both versions of sealed lead-acid batteries is identical in that they are both recombinant-type batteries that have positive limited plate groups that operate in the oxygen cycle. In all types of lead-acid batteries, a lead oxide positive plate and a sponge lead negative plate are placed in a dilute mixture of sulfuric acid. The voltage difference that is produced by this electrochemical couple causes electrons to flow from one plate to the other plate when the plates are connected. This electron flow causes a chemical reaction inside of the battery. The chemical reaction will be discussed in more detail later in this section. When a lead acid battery is discharged, much of the sulfuric acid electrolyte is changed to water and both of the plates are reduced to lead sulfate. When a lead-acid battery is subsequently charged by forcing electrons to flow in the opposite direction, all of the chemical reactions are reversed.



During recharge, oxygen is released at the positive plate, and, when the negative plate is fully charged, it releases hydrogen. These gases are formed from the decomposition of the water that is mixed with the sulfuric acid. The gases must be vented out of the battery room, and they must be properly dispersed because of the explosive nature of hydrogen. In flooded lead-acid batteries, this water loss must be replaced on a regular basis to prevent the plate from drying out. In sealed lead-acid batteries, the negative plate is designed to be what is known in the industry as "positive limited" because the chemical reaction of discharging is stopped when the positive plate is exhausted. When a sealed lead-acid battery is recharged, the negative plate never reaches its full charge condition; therefore, hydrogen gas is not released. The oxygen that is released at the positive plate travels through the void paths that are in the separator to the negative plate. At the negative plate, the oxygen combines with the lead in the negative plate to form lead dioxide. The lead dioxide reacts with the sulfuric acid that is in the electrolyte to form lead sulfate and water. This water replaces the water that was consumed at the positive plate to make the oxygen; therefore, water never needs to be added to the battery and the case can be sealed. The oxygen that is generated at the positive plate creates a positive pressure inside of the battery that is two to three pounds above the ambient pressure. Because of this positive pressure, sealed lead-acid batteries must contain a relief valve to prevent overpressurization of the battery in the event of a malfunction such as a runaway charger. The relief valves are designed to automatically reseal once the pressure returns to normal. The construction of the gelled electrolyte version of sealed lead-acid batteries is similar to the construction of flooded lead-acid batteries with a few key differences. In the gelled electrolyte version, the dilute sulfuric acid mixture is blended with silica to form a gel or a paste-like substance. This gelled electrolyte is then used with a pasted plate element that is complete with a microporous separator. The primary difference between a pasted plate element for a flooded lead-acid battery and the pasted plate element for sealed lead acid battery is that the element that is used in the sealed lead-acid battery is positive limited. The other key difference is that the sealed lead-acid battery is constructed with a relief valve rather than with a flash arrestor. In the AGM version, the positive limited element is constructed without the microporous separator. Instead, the positive plates are separated from the adjoining negative plates by a fiberglass mat. The fiberglass mat absorbs the liquid electrolyte so that no free liquid exists inside of the battery. The very fine micro fiber construction of the mat and its relatively thin (approximately 1/8 inch) construction results in a very low internal cell impedance. Because of the AGM version's construction, it is generally smaller, lighter, and more energy-efficient for short discharge periods than the gelled electrolyte version. However, the AGM version is more prone to plate shorting as a result of small lead filaments bridging the thin glass mat separator.



The following are the major advantages and disadvantages of sealed lead-acid batteries: Advantages • Long service life (15 to 20 years) in float service. • No maintenance. • Reduced battery room/ventilation requirements. • Vertical or horizontal mounting.

Disadvantages • High cost. • Short service life (< 10 years) in cycling service. • Moderate self-discharge rates (3%/month). • Not suitable for deep or frequent discharges.

The development of sealed lead-acid batteries for large stationary storage applications is a relatively recent (within the last ten years) technology and, as such, these batteries are more expensive than their lead calcium and lead antimony flooded cell counterparts. The cost of sealed lead-acid batteries is similar to the cost of Plante lead-acid batteries. Because most sealed lead-acid batteries for stationary storage applications use a lead calcium grid, their cyclic performance is similar to the cyclic performance of the flooded lead calcium batteries that were previously discussed. Sealed lead acid batteries do not perform well in frequent or deep cycle application; these batteries should only be used in float charge, shallow cycle applications. Nickel-Cadmium Batteries The nickel-cadmium battery also is a group of electrochemical cells that are connected in series to generate a nominal dc voltage that would supply power to a suitably connected electrical load. The number of cells that are connected in series determines the nominal voltage rating of the battery. The amount of active material that is contained in an individual cell is the factor that determines the discharge capacity rating of the cell; the rated capacity of the individual cells is the rated capacity of the entire battery. Connecting the individual cells in series does not increase the capacity rating of the battery. The electrochemical couple that is used to form nickel-cadmium cells is configured through placement of a nickel hydroxide positive electrode and a metallic cadmium negative electrode in a dilute potassium hydroxide electrolyte. This electrochemical couple produces a nominal cell voltage of approximately 1.2 volts.



The following types of nickel-cadmium stationary storage batteries will be discussed in this section: • •

Pocket Plate Sintered Plate

Pocket Plate

Pocket plate nickel-cadmium batteries get their name from the "pocket" strip that holds the active material of the plates. The pockets are made from very thin, finely perforated strips of steel that are formed into shallow "U" channels as shown in Figure 5. The positive active material, which consists of hydrates of nickel oxide and graphite, and the negative active material, which consists of cadmium oxide and a small amount of iron oxide, are placed into the open "U" channel. The open "U" channel is then covered with a similar strip of perforated steel and the two strips are crimped together to form a perforated pocket. A number of these perforated pockets are interlocked edge-to-edge and then cut to the approximate finished plate width. The interlocked pockets are then rolled to compress the active material and to form longitudinal indentations in the plate. Each plate group is then bolted or welded together. The two (positive and negative) plate groups are interleaved and insulated with plastic rod or mat separators that are inserted into the longitudinal grooves of the plates. The insulated plate group or element is then inserted into a plastic container or jar. After the mechanical assembly is complete, the cells are put through an electrochemical formation process to convert the active materials to their charged condition.



Pocket Plate Construction Figure 5



Figure 6 shows a cutaway view of a typical pocket plate nickel-cadmium battery. The following components are identified in Figure 6:

• • • • • • •

Vent Cap/Flame Arrestor Cell Container or Jar Plate Tab Plate Group Bus Separating Grids Plate Frame Plate

Typical Pocket Plate Nickel-Cadmium Battery



Figure 6



The vent cap/flame arrestor allows the gases that are generated during normal charging and discharging to escape from the battery, but it prevents the entry of sparks and foreign material. The vent cap/flame arrestor can be removed to allow access to the interior of the cell. The cell container or jar is made out of translucent polypropylene. The cell container or jar is the enclosure that holds the individual components of the cell and that contains the liquid potassium hydroxide (KOH) electrolyte. The plate tabs are spot welded to the plate frame and to the upper edge or the pocket plates. The plate tabs provide the electrical connection between the plates and the plate group bus. The plate group bus connects the plate tabs to the terminal posts of the battery. The plate tabs and the terminal posts are welded to opposite sides of the plate group bus. The separating grids separate the plates and insulate the plate frames from each other. The separating grids are porous, and they allow the free circulation of the KOH electrolyte between the plates. The plate frames seal the plate pockets, and they serve as current collectors. The plates in this particular battery have horizontal pockets of double-perforated steel strips. Most manufacturers make three general pocket plate designs: thin plates, medium plates, and thick plates. The thin plate design has a low internal resistance, and it is used in high rate, short duration (less than 30 minutes) discharge cells. The medium plate design has an internal resistance that is similar to the internal resistance of general purpose lead-acid cells. This plate design is used in median rate, median duration (from 30 minutes to 2 hours) discharge cells. The thick plate design has a high internal resistance, and it is used in very low rate, long duration (more than two hours) discharge cells. All vented nickel-cadmium batteries have several general advantages over their flooded lead acid counterparts. One of these advantages stems from the KOH electrolyte that is used in nickel-cadmium batteries. The freezing point of KOH electrolyte with a typical specific gravity of 1.190 is -32 oC (-25 oF). Because the specific gravity of the electrolyte in a nickelcadmium battery remains relatively constant from the fully charged state to the fully discharged state, the freezing point also remains relatively constant. In contrast, the freezing point of a fully discharged lead-acid battery essentially is the same as the freezing point of water or 0 oC (32oF); therefore, lead acid batteries are much more likely to freeze than nickelcadmium batteries. Nickel-cadmium batteries also make a significantly higher percentage of their current available at lower temperatures than lead-acid batteries.



Another general advantage of nickel-cadmium batteries is that they require much less maintenance than flooded lead-acid batteries. Nickel-cadmium batteries can go years without being watered, and they will not deteriorate when they are left in a discharged condition. Nickel-cadmium batteries have successfully been returned to service after being left "on-theshelf" for 10 to 15 years. The other general advantages of nickel-cadmium batteries include a more rugged and durable construction (i.e., relatively immune to vibration, shock, and overcharge currents), a long cycle life, and minimal gas generation on charge and discharge. Also, the gaseous vapors that are emitted from nickel-cadmium batteries are not corrosive to ferrous metals. The only general disadvantage of nickel-cadmium batteries to flooded lead-acid batteries is the high cost of nickel-cadmium batteries. The cost (dollars/kilowatt-hour) of nickelcadmium batteries is four to ten times the cost of flooded lead-acid batteries. Also, because cadmium is more difficult and expensive to recycle and/or dispose of than lead, the disposal cost of expended nickel-cadmium batteries is higher than it is for expended lead-acid batteries. In North America, large nickel-cadmium storage batteries are actually being phased out of production as a result of the disposal issues that surround cadmium. The advantages of pocket plate nickel-cadmium batteries over sintered-plate nickel-cadmium batteries are that pocket plate nickel-cadmium batteries have a lower cost, they have a long cycle life, and they do not suffer a "memory" effect on shallow discharges. The major disadvantage of pocket plate nickel-cadmium batteries is that they only have about 50% of the energy density of sintered-plate nickel-cadmium batteries. In contrast to flooded stationary lead-acid batteries, pocket plate nickel-cadmium batteries have a long cycle life in addition to a long calendar life. Under normal operating conditions, pocket plate nickel-cadmium batteries can deliver as many as 2000 equivalent full charge cycles. The calendar life of pocket plate nickel-cadmium batteries ranges from 15 to 25 years. Because of the large number of cycles that this battery can withstand (more than 80 equivalent full charge cycles per year over a 25 year calendar service life), cycling has little or no effect on the calendar service life of pocket plate nickel-cadmium batteries. Sintered-Plate

Because Saudi Aramco does not permit the use of sintered-plate nickel-cadmium batteries, this section is intended only for general information purposes. The primary applications of sintered-plate nickel-cadmium batteries are those that require high-power discharge service in a lightweight compact package such as aircraft turbine engine starting circuits.



Sintered-plate nickel-cadmium batteries are constructed similarly to pocket plate nickelcadmium batteries. The major difference between the two batteries is the way in which the electrodes (positive and negative plates) are designed. In the sintered-plate design, the active materials are impregnated into a porous nickel coating that is applied to an iron grid or charge collector. Sintered-plate nickel-cadmium batteries have nickel oxyhydroxide positive plates and cadmium hydroxide negative plates. The plates are separated by non-conductive, porous materials that act as a gas barrier and an electrical separator. The electrolyte is a dilute KOH solution, and it completely covers the plates and separators. Figure 7 shows the construction of a typical sintered-plate nickel-cadmium battery. The battery consists of a plate pack that contains the positive plates, the negative plates, and the separators. A terminal comb is placed over the negative plate tabs to connect the negative plates. A separate terminal comb is placed over the positive plate tabs to connect the positive plates. The assembled plate pack is then placed in the cell container. A cell cover is placed on top of the cell container, and it is sealed to the container. A terminal is connected to the positive terminal comb and to the negative terminal comb. The terminals are used to connect the battery to the external load. The removable vent cap allows excess gases to escape and a means to add electrolyte.



Typical Sintered-Plate Nickel-Cadmium Battery Figure 7



The advantages and disadvantages of sintered-plate nickel-cadmium batteries are shown in Figure 8.

Advantages and Disadvantages of Sintered-Plate Nickel-Cadmium Batteries Figure 8



As was noted in Figure 8, the sintered-plate nickel-cadmium battery is more expensive than the vented pocket plate battery and many times more expensive than the sealed lead-acid battery. The average calendar life of a sintered-plate nickel-cadmium battery ranges from three to ten years, and the cycle life ranges from 500 to 2000 equivalent full charge cycles. The cycle life and calendar life are strongly influenced by the method of operation, the ambient temperature conditions, and the depth of discharge. The best battery life performance is obtained from operation at normal temperatures and at moderate (50% average) discharge depths. The life of sintered-plate nickel-cadmium batteries drops when the battery is subjected to frequent deep or shallow discharges. Operational Characteristics Each type (lead-acid or nickel-cadmium) of battery has a unique set of operational characteristics that are based on the construction and configuration of the electrochemical couple that is used in the battery. The following operational characteristics of batteries will be discussed in this section: • • •

Electrochemical Reaction Charge Discharge Characteristics Effects of Temperature on Battery Life and Capacity

Electrochemical Reaction

The electrical energy that secondary batteries are capable of delivering is derived from the electrochemical reaction that occurs between two electrically dissimilar metals or metallic compounds. In the case of lead-acid batteries, the electrolyte also takes part in the electrochemical reaction. The electrochemical reaction is essentially reversible and electrical energy is consumed to restore the battery to a charged condition. The sections that follow provide an explanation of the electrochemical reactions that occur during the discharge and charge of the following types of secondary storage batteries: • •

Lead-Acid Nickel-Cadmium

Lead-Acid - In a fully charged lead-acid battery, the positive electrode material is lead dioxide

(PbO2), the negative electrode material is pure sponge lead (Pb), and the electrolyte is a mixture of sulfuric acid (H 2SO 4) and water (H 2O). As the battery is discharged, both of the electrodes are converted to lead sulfate (PbSO 4) and the electrolyte is consumed, which produces water; the process reverses when the battery is charged. This electrochemical reaction is called the "double-sulfate" reaction.



Figure 9 shows the discharge electrochemical reaction that occurs in a lead-acid battery. The initial reaction that occurs on discharge is an ionization process. The 2H 2SO 4 breaks down into 2SO2-4 and 4H+ through the ionization process. The PbO 2 breaks down to 4OH - and Pb 4+ . After the ionization process, the current producing process begins to produce usable electric current. The Pb of the negative electrode reacts with the SO 2-4 ions that are in the electrolyte. The product of this reaction is Pb 2+ and 2e; the Pb 2+ then combines with the SO 2-4 ions to produce PbSO 4. PbSO4 is the discharged composition of the negative electrode. At the positive electrode, the Pb 4+ combines with the SO 2-4 ions that are in the electrolyte and with 2e from the load to produce PbSO 4. PbSO4 also is the discharged composition of the positive electrode. The 4OH - ions from the positive electrode combine with the 4H + ions that are in the electrolyte to produce 4H 2O. This discharge reaction continues until the active materials are effectively depleted.

Discharge Electrochemical Reaction Figure 9



After the battery is discharged, a reverse polarity electric current must be applied to the battery to reverse the electrochemical reaction, which restores the battery to a charged condition. Figure 10 shows a graphical representation of the electrochemical reaction that occurs inside of a lead-acid cell during a battery charge. When a reverse polarity current is applied to the electrodes, the PbSO 4 that comprises both electrodes ionizes to form Pb 2+ ions and SO 2-4 ions. Also, the water (4H 2O) that is in the electrolyte ionizes to form 4H + ions and 4OH - ions. After ionization occurs, the Pb 2+ ions of the negative electrode absorb 2e, which converts the negative electrode back to Pb. The SO2-4 ions from both of the electrodes combine with the 4H + ions that are in the electrolyte, which forms 2H 2SO 4. The Pb2+ ions of the positive electrode combine with the 4OH - ions that are in the electrolyte, which converts the positive electrode back to PbO 2 and forms 2H 2O. This charge reaction continues until the previously depleted active materials are returned to their original composition. The following is the overall discharge/charge equilibrium equation for lead-acid batteries: Pb + PbO 2 + 2H 2SO 4 _ 2PbSO 4 + 2H 2O + 2e

Electrochemical Reaction During a Battery Charge Figure 10



When a lead-acid battery approaches its full charge condition, the majority of the PbSO 4 has been converted to Pb or PbO 2, which causes the battery voltage on charge to rise above the gassing voltage. The overcharge reactions then begin. The overcharge reactions result in the production of hydrogen and oxygen (i.e., gassing) and the resultant loss of water. The following are the equations for the overcharge reactions: Negative Electrode Reaction: 2H + + 2e _ H 2 Positive Electrode Reaction: H 2O - 2e _ 1/2O 2 + 2H + Overall Reaction: H 2O _ H 2 + 1/2O2 In sealed lead-acid batteries, the above reactions are controlled by design to prevent hydrogen evolution and the loss of water through recombination of the evolved oxygen with the negative plate. Nickel-Cadmium - In a fully charged nickel-cadmium battery, the positive electrode

material is nickel oxyhydroxide (NiOOH), the negative electrode material is cadmium (Cd), and the electrolyte is a mixture of potassium hydroxide (KOH) and water. When nickel-cadmium batteries are discharged, the active materials that are contained in the electrodes change in oxidation with no deterioration in the physical state. The active materials are present only as solids that are highly insoluble in the KOH electrolyte. Also, the KOH electrolyte does not participate in the electrochemical reaction; the electrolyte only acts as a current carrying medium. During a discharge, the cadmium metal of the negative electrode oxidizes to cadmium hydroxide (Cd(OH) 2) and releases electrons to the external circuit. This portion of the electrochemical reaction is shown in the following equation: Cd + 2OH- _ Cd(OH) 2 + 2e At the positive electrode, the NiOOH is reduced to the lower valence state of nickel hydroxide (Ni(OH) 2) by accepting electrons from the external circuit. This portion of the electrochemical reaction is shown in the following equation: 2NiOOH + 2H 2O + 2e _ 2Ni(OH) 2 + 2OH-



After the battery is discharged, a reverse polarity electric current must be applied to the battery to reverse the electrochemical reaction, which restores the battery to a charged condition. When a reverse polarity current is applied to the electrodes, the Cd(OH) 2 of the negative plate accepts electrons and returns to its original state as shown by the following equation: Cd(OH) 2 + 2e _ Cd + 2OH At the positive electrode, the 2Ni(OH) 2 is oxidized back to the its original higher valence state of NiOOH as shown by the following equation: 2Ni(OH) 2 + 2OH- _ NiOOH + 2H 2O + 2e The following is the overall discharge/charge equilibrium equation for nickel-cadmium batteries: Cd + 2NiOOH + 2H 2O _ Cd(OH) 2 + 2Ni(OH) 2 Charge/Discharge Characteristics

Figure 11 shows the way in which the following lead-acid battery characteristics change during a constant current discharge and subsequent charge: • • •

Specific gravity Ampere-hours discharged/charged Volts per cell

When a lead-acid battery is discharged at a constant, the specific gravity linearly decreases in proportion to the number of ampere-hours that are discharged. In contrast, the volts per cell remain relatively constant at the beginning of the discharge and then the volts per cell decrease at an increased rate as the voltage approaches the end voltage of 1.75 volts. When the battery reaches its end voltage, the discharge should be stopped and the battery should be recharged. When a lead-acid battery is placed on a constant rate (current) charge, the volts per cell have an initial step jump increase, and then the volts per cell gradually rise and level off toward the end of the charge. The specific gravity increase during the battery charge is not linear and it lags behind the number of ampere-hours charged. The reason that the specific gravity increase lags the number of ampere-hours charged is because complete mixing of the electrolyte does not occur until gassing begins. Once gassing begins, the specific gravity quickly rises to its full charge level.



Lead-Acid Battery Charge/Discharge Characteristic Curves Figure 11

Figure 12 shows a typical charge/discharge characteristic curve for a vented nickel-cadmium battery. When a constant current discharge is placed on a fully charged nickel-cadmium battery, the battery voltage initially drops as it begins to supply current to the load. After the initial drop in voltage, the voltage remains essentially constant until about 90% to 95% of the ampere-hours have been discharged. At this point, the volts per cell quickly drop to the final voltage of 1.1 volts. When the battery reaches its end voltage, the discharge should be stopped and the battery should be recharged. When a vented nickel-cadmium battery is charged at a constant rate, the volts per cell have an initial step jump increase and then gradually rise and level off toward the end of the charge. The charge curve for a vented nickel-cadmium battery is similar to the charge curve for a lead-acid battery except that the volts per cell values are lower.



Because the specific gravity of a nickel-cadmium battery remains essentially constant from fully charged to fully discharged, the curve for the specific gravity of a nickel-cadmium battery is not shown in Figure 12.

Nickel-Cadmium Battery Charge/Discharge Characteristic Curves Figure 12



Effects Effects of Temper atur e on Battery Ca pacity and Service Life Life

The temperature at which a lead-acid battery is operated has a definite effect on its capacity and its service life. Figure 13 shows the effects of temperature on the capacity of a lead-acid battery at several different discharge rates. For each discharge rate that is shown (3 to 8 hour rate, 0.5 to 3 hour rate, and 0.5 hour rate), the battery delivers its rated capacity capacity (100%) at an o electrolyte temperature of 25 C, which is the optimum operating temperature for a lead-acid storage battery. battery. As the operating operating temperature drops below 25 oC, the % rated capacity of the battery also decreases. decreases. The decrease is due to reduced reduced chemical activity and to increased increased internal resistance. As the operating temperature rises above 25 oC, the % rated capacity of the battery also increases; however, such operation should be avoided as the high temperatures also cause a severe reduction in service life. Operation of a lead-acid storage at an electrolyte temperature of 35oC reduces the service life by 50%.

Effect of Temperature on Lead-Acid Battery Capacity Figure 13



High and/or low electrolyte electrolyte temperatures have a much smaller effect on nickel-cadmium nickel-cadmium batteries than they do on lead-acid batteries. Pocket plate nickel-cadmium storage batteries that contain standard electrolyte electrolyte concentrations can be used at temperatures temperatures as low as -20 oC with little loss of capacity. Batteries that contain concentrated electrolyte can be used at temperatures temperatures as low as -50 oC. Pocket plate nickel-cadmium storage batteries can also be continuously operated at elevated temperatures with little or no loss of service life. Generally, these batteries can operate with electrolyte temperatures that range from 45 to 50 oC with no long term detrimental effects. Battery Applications A number of factors must be considered to select the best battery for a particular application. No single battery provides provides optimum performance performance under all operating operating conditions. The characteristics of each available battery must be weighed against the total equipment requirements, requirements , and the battery that is selected must best fill these requirements. requirements . Generally, the following factors are considered in the selection process: • • • • •

Ambient temperature Life cycle cost Required am ampere-hour ca capacity an and du duty cy cycle Frequency of required maintenance Available space

Because of the effects of electrolyte temperature on the capacity and the life of lead-acid batteries, the ambient temperature of the battery installation site must be considered in the battery selection process. If the battery will be required to operate in low ambient temperatures, a lead-acid battery may still be satisfactory; however, a larger cell size would be required to make up for the decreased ampere-hour capacity of lead-acid batteries batteries at low temperatures. temperatures . If the battery will be required to operate in high ambient temperatures, a nickelcadmium battery may be the only acceptable choice because of the severe service life reduction that is suffered by lead-acid batteries when they are operated in high ambient temperatures. The life cycle cost of the different batteries that could be used in a given installation must be determined to realistically compare the cost of different battery types. The life cycle cost of a given battery is the initial capital cost of the battery plus the anticipated cost of maintenance over the battery's anticipated service life, divided by the anticipated calendar service life or number of equivalent full charge cycles, whichever is applicable.



The required ampere-hour capacity and duty cycle must be known to determine the required cell size for the application. This topic is discussed in more detail in the next section of this Module ("Determining Battery Size for use in Typical Saudi Aramco Applications"). Applications"). The frequency of required maintenance must be considered from the standpoint of both cost and practicality. practicality. If a battery is going to be installed installed in a remote, unmanned location, location, a high capital cost battery that requires minimal maintenance may have a lower overall cost and be more practical than a low capital cost, high maintenance battery. battery. The amount of space that is available must be considered to ensure that the chosen battery will fit in the installation. If the installation space is limited, the additional cost of a compact, high-performance battery is justified. The following are typical examples of applications in which each type of battery might be used: •

Plan Plante te lead lead-a -aci cid d bat batte teri ries es are are use used d for for UPS UPS ins insta tall llat atio ions ns that that have have air air conditioned battery battery rooms. This is one of the few applications applications in which the long service life of the Plante battery justifies its cost.

•

Lead Lead calc calciu ium m bat batte teri ries es are are use used d in in air air cond condit itio ione ned d app appli lica cati tion onss in in whi which ch the battery is only expected to be subjected to infrequent and shallow discharges. Lead calcium batteries are not suitable for frequent and/or deep discharge services. services. Lead-calcium batteries batteries are suitable suitable for many UPS installations, emergency lighting installations, and telephone exchanges.

•

Lead Lead ant antim imon ony y batt batter erie iess are are used used in in air air cond condit itio ione ned d appl applic icat atio ions ns in in which the battery is expected to be subjected to frequent and/or deep discharges but also in which the high cost of Plante batteries cannot be justified.

•

Seal Sealed ed lea leadd-ac acid id bat batte teri ries es are are use used d in app appli lica cati tion onss that that req requi uire re no no maintenance or that require the battery to be installed in other than a horizontal configuration. configuration.

•

Beca Becaus usee nick nickel el-c -cad admi mium um batt batter erie iess are are gene genera rall lly y more more expe expens nsiv ivee than than a comparable lead-acid battery, battery, they should not be used in air conditioned spaces. Because the temperature temperature has only a small effect effect on the nickelcadmium battery's life and capacity, the nickel-cadmium battery is ideal for installation in high temperature, remote locations.



DETERMINING BATTERY SIZE FOR USE IN TYPICAL SAUDI ARAMCO APPLICATIONS After a specific type of battery is selected for a particular application, the size of the battery must be determined. Determination Determinati on of the proper size will ensure that the battery can supply power to the connected load load for the specified duration duration of time. This section provides provides information on the following topics that are pertinent to determining the correct battery size: • • • •

Load Criteria Duty Cycle Battery Voltage Determining Battery Capacity

Load Cr iter iter ia Accurate specification specification of battery size depends on an accurate definition of the system's load. The following data must be considered for each item of connected electrical equipment to accurately define the system's load: • • • •

Voltage Range (Window) Current or kW Draw Loa Load Cla Classi ssific ficati ation (Dur (Duraatio tion of Opera perattion) on) Frequency of Use

Voltage Range (Window)

Stationary battery battery systems can be designed designed to meet almost any desired voltage voltage rating. Most dc-powered electrical equipment falls within one of the following major dc voltage groups: •

6/12 vo volts (e (emergency li lighting un units)

•

24 vol volts ts (al (alar arm m syst system ems, s, eng engin inee cran cranki king ng,, comm commun unic icat atio ions ns sys syste tems ms))

•

32 volt voltss (em (emer erge genc ncy y lig light htin ing g sys syste tems ms,, eng engin inee cra crank nkin ing, g, elec electr tric ic cloc clock k systems)

•

48 volt voltss (swi (switc tchg hgea earr syst system ems, s, tele teleph phon onee syst system ems, s, micr microw owav avee syst system ems, s, engine cranking)

•

120 120 vol volts ts (swi (switc tchg hgea earr sys syste tems ms,, eme emerg rgen ency cy ligh lighti ting ng syst system ems, s, boil boiler er flam flamee control, communication systems, telemetering, supervisory control systems, fire alarm systems, UPS systems, large engine cranking)

•

240 240 volt voltss (sw (swit itch chge gear ar syst system ems, s, UPS UPS sys syste tems ms,, larg largee eng engin inee cra crank nkin ing) g)

•

Higher voltages (UPS systems)



The battery system voltage is based on the number of cells that are connected in series and the nominal voltage of each cell; however, battery systems are normally maintained at voltages that are higher than the nominal nominal voltage of the system. system. For example, a battery system system that has a nominal system voltage voltage of 120 volts is normally operated operated at 130 to 135 volts. This higher voltage is due to the fact that most battery systems are always operated in a float charge condition, and the float charge voltage is always higher than the nominal system voltage. Most dc-powered equipment items are designed to operate on a fairly broad range of dc supply voltages for the following reasons: •

To acco accomm mmod odat atee the the gra gradu dual al decl declin inee of of bat batte tery ry volt voltag agee as as the the bat batte tery ry discharges.

•

To acco accomm mmod odat atee the the vol volta tage ge incr increa ease sess tha thatt occ occur ur when when the the bat batte tery ry is on charge.

The minimum and the maximum voltage that is permissible for each item of connected electrical equipment must be known to properly size the battery in terms of voltage. The actual determination of battery voltage is discussed in more detail later in this module. Curr ent ent or kW Draw

Each item of electrical equipment is assigned a nominal current rating (ampere rating) or a kW rating by its manufacturer. manufacturer. If the equipment is assigned assigned a nominal current current rating, this rating can be used as the equipment's contribution contribution to the overall load on the battery for the period of time that the equipment equipment operates. If the equipment is assigned assigned a nominal kW rating, this rating must be divided by the battery system voltage to determine the equipment's contribution to the overall load on the battery for the period of time that the equipment operates. When making this calculation, calculation, the engineer must note that, that, as the battery voltage voltage decreases during the discharge, the current load must increase to maintain the same kW. In addition to the normal ampere rating, some electrical equipment equipment has another current factor that is seldom rated by the manufacturer but that is vitally important to battery size. This factor is the temporary high ampere demand or inrush current that is imposed on the power source (battery) (battery) when equipment such as electric electric motors are started. started. Inrush demands must be determined or estimated on the high side to ensure that the battery voltage does not drop below its specified minimum value during the inrush.



Load C lassification (Dura tion of Opera tion)

For Saudi Aramco installations, the individual dc loads that are supplied by a battery are classified as follows: • • •

Continuous loads Essential loads Momentary loads

Continuous loads are indicating lights, alarm systems, and other devices that are necessary for personnel safety. The following are the minimum continuous load durations that must be provided: •

Twelve hours for the continuous loads that are located in attended substations or other similar locations.

•

Eighteen hours for the continuous loads that are located in unattended substations in on-shore locations.

•

Twenty-four hours for the continuous loads that are located in unattended off-shore locations.

Essential loads are critical motors, emergency lighting, selected communication equipment, emergency shutdown systems (ESD), and any loads that are determined to be life critical by the Loss Prevention Department. Regardless of the location of the load, the minimum essential load duration that must be provided is three hours. Momentary loads are random, short duration loads that are considered coincident with the highest load requirement. Generally, momentary loads provide close or trip power for switchgear and generator field flashing. UPS systems may also require a momentary load at the end of the discharge to furnish power for ESD systems to energize motor operated valves.



The design duration of momentary loads is different for lead-acid and nickel-cadmium batteries. Because of the time that is required for the battery to respond to an abrupt load change, the momentary loads for lead-acid batteries system have a design duration of one minute. The design duration for nickel-cadmium batteries is one second. The faster response of nickel-cadmium batteries can result in smaller ampere-hour ratings. The loads that are included in the three listed categories are not a comprehensive list of all the possible dc loads at a given installation. The Electrical Engineer must carefully analyze each system to be sure that all possible loads and load variations have been included. Frequency of Use

Because some of the electrical equipment that is supplied by the battery may be energized more than once during the battery discharge, the anticipated frequency of such usage must be specified. If the frequency of use varies dependent on equipment positions or status when the battery discharge begins, the maximum number of possible operations should instead be specified. Such a specification is necessary to ensure that the battery will have sufficient capacity to handle a worst-case situation.



Duty C ycle The duty cycle of a battery is defined as the load currents that a battery is expected to supply for specified periods of time. Accurate information about the anticipated duty cycle of a new battery is needed to properly size the battery. Selection of the proper size of battery depends not only on the nominal current rating or the kW draw and the duration of each load (i.e., the duty cycle), but it also depends on the sequence in which the load are energized. Careful scheduling of the load sequence in the duty cycle helps keep the required cell size to a minimum and reduces the cost of the installation. Most stationary battery installations are multi-load systems rather than single-load systems. When a variety of loads are connected to a battery, sudden increases and decreases in current demands are imposed on the battery. If the high-current loads can be scheduled to energize at the beginning of the duty cycle rather than at the end of the duty cycle, a smaller sized battery can be used. However, if the load is random in nature and could occur at any time during the duty cycle, the best practice is to assume that the load will energize at the most limiting point in the duty cycle when its effect will be the most severe. A duty cycle diagram is a tool that aids in the analysis of the duty cycle and in the determination of the required battery capacity. A duty cycle diagram shows the total required battery current at any given time in the duty cycle. To prepare such a diagram, all of the loads that are expected during the duty cycle are tabulated along with their anticipated start and stop times. The loads that have known start and stop times are plotted on the duty cycle diagram as they would occur. The remainder of the loads should be plotted through use of the following guidelines: •

If the start time for a load is known but the stop time is indefinate, the load is assumed to be continuous for the remainder of the duty cycle.

•

If the load can occur at random, the load is assumed to occur at the most critical point in the duty cycle to simulate the worst-case condition. The most critical point in the duty cycle is the point that controls the size of the battery that is needed.



Figure 14 shows an example of the duty cycle diagram that would be plotted from the following hypothetical loads: •

L1 represents 50 amps of continouus emergency lighting load for three hours.

•

L2 represents 100 amps of momentary switchgear operations load and 250 amps of momentary motor starting load.

•

L3 represents 50 amps of noncontinuous motor load for one and a half hours.

•

L4 represents 100 amps of noncontinuous load that starts after 30 minutes and stops after one hour of operation.

•

L5 represents 50 amps of momentary switchgear operations load that occurs during the last minute of the duty cycle.

•

L6 represents four 25 amp random momentary loads that can occur at any time during the duty cycle.



Duty Cycle Diagram Figure 14



Battery Voltage The voltage at which a battery system operates is not constant; rather, it fluctuates between minimum and maximum fixed values dependent on the state of charge. The range of voltages that lies between the minimum system voltage and the maximum system voltage is the voltage operating "window" for the battery system. The size (number of cells) of the battery must be selected to ensure that the battery operates inside of the voltage window. For batteries that supply dc systems, the minimum and the maximum battery system voltages are based on the voltage rating of the most limiting load. The minimum system voltage is selected to ensure that sufficient voltage is available to operate the loads at the end of the battery duty cycle. The maximum system voltage is selected to ensure that the voltage rating of the loads will not be exceeded when the battery is being recharged. The following are the minimum and the maximum battery system voltages for Saudi Aramco dc systems: Nominal System Voltage 12 vdc 24 vdc 48 vdc 120/125 vdc 240/250 vdc 360/375 vdc

Minimum System Voltage 10.5 vdc 21.0 vdc 42.0 vdc 105 vdc 210 vdc 315 vdc

Maximum System Voltage 15.5 vdc 30.0 vdc 58 vdc 143 vdc 286 vdc 429 vdc

For batteries that supply UPS systems, the minimum and the maximum battery system voltages are based on the allowable dc input voltage ratings of the UPS system. The allowable dc input voltage ratings are specified by the manufacturer of the UPS system. For batteries that supply a combined dc and UPS system load, the minimum and the maximum system voltages for dc systems should be used. Because the battery system voltage is the product of the individual cell voltages and the number of cells that are connected in series, the minimum and the maximum battery system voltage values are used to determine the minimum and the maximum number of cells that can be used in a battery installation. The following equation (included in Work Aid 2) can be used to calculate the minimum number of cells that can be used in an installation: For most applications, the final volts per cell for lead-acid batteries is 1.75, and the final volts per cell for nickel-cadmium batteries is 1.1 volts. If justified by valid engineering and economic reasons, other final volts per cell values could be used. The following equation (included in Work Aid 2) can be used to calculate the maximum number of cells that can be used in an installation:



The maximum cell voltage on charge for lead-acid batteries is the equalizing voltage that is recommended by the manufacturer. If the manufacturer's data are not available, a value of 2.33 volts per cell can be assumed. The maximum cell voltage on charge for nickel-cadmium batteries is the charging voltage that is recommended by the manufacturer. If the manufacturer's data are not available, a value of 1.55 volts per cell can be assumed. After the minimum and the maximum number of cells are known, the actual number of cells can be specified. The actual number of cells that are specified can be any number that lies between the minimum and the maximum number. For consistency and ease of specification, SAES-P-103 contains the following information that lists the number of lead-acid or nickelcadmium cells that should be specified for various nominal battery system voltages: Nominal Battery System Voltage (vdc) 12 24 48 120/125 240/250 360/375

Number of Cells

Lead-Acid 6 12 24 60 120 180

Nickel-Cadmium 10 19 37 92 184 276

The equations serve as a second check to verify that the number of cells that are listed is correct for the application. For example, if a new lead-acid battery is needed to supply emergency power for a dc system with a nominal voltage of 120 vdc, the number of cells that are required is determined as follows: •

From the table that is in SAES-P-103, 60 lead-acid cells are required for a nominal battery system voltage of 120 vdc.

The number of cells that were determined from the table can be verified as follows: The above calculations verify that 60 lead-acid cells are adequate for this installation.



Determ ining Batter y Capa city The capacity of a secondary storage battery is usually expressed in amperes and time. For example, a battery capacity can be expressed as 42.5A for eight hours or 340 ampere-hours (Ah) at the eight hour rate. Because the temperature of the electrolyte and the minimum allowable cell voltage also affect the number of Ah's that a given battery can deliver, these values must also be included as part of the battery's capacity rating. For example, a completely defined battery capacity is 42.5A for eight hours at an electrolyte temperature of 25 oC to a final cell voltage of 1.75 vdc. The following is a summary of relationships that exist between the various elements of a battery capacity rating: •

As the discharge rate (amperes) of a battery increases above the standard rate, the length of time that elapses before the final cell voltage is reached decreases.

•

As the discharge rate (amperes) of a battery decreases below the standard rate, the length of time that elapses before the final cell voltage is reached increases.

•

If the electrolyte temperature is less than the standard rated electrolyte temperature and the battery is still discharged at its standard rate, the final cell voltage rating will be reached before the standard rated time elapses.

•

If the final cell voltage that is allowed increases, the number of amps that the battery can deliver for the standard rated time period decreases or the length of time that the battery can deliver its standard rated amps decreases.

•

If the final cell voltage that is allowed decreases, the number of amps that the battery can deliver for the standard rated time period increases or the length of time that the battery can deliver its standard rated amps increases.

Each stationary storage battery manufacturer generally designs more than one series of cell types that vary in plate thickness and separation, in the number of plates, and in the size of the current carrying parts. These variances result in large differences in the performance of different cells that have similar nominal Ah capacities. For example, the following list shows six different commercially available cells from different manufacturers:



Discharge in Amperes at 25 oC to 1.75 v/c (Lead-Acid) and 1.10 v/c (Nickel-Cadmium)

Plante Lead-Calcium Lead-Antimony Nickel-Cadmium (L) Nickel-Cadmium (M) Nickel-Cadmium (H)

Ah Capacity for 8 Hours 760 752 752 760 736 744

8 Hours 95 94 94 95 92 93

3 Hours 190 188 202 237 232 241

1 Hour 330 369 411 454 540 666

1 Minute 500 910 964 1140 1780 3180

The list shows that although all of the batteries have similar Ah capacities at the eight hour rate, the batteries have very different ampere capabilities for shorter duration discharges. If one of the batteries from the table was specified for a given application solely on the basis of its Ah rating at the eight hour rate, the battery could be too small or too large for the application by a factor of two. Because of all of the variables that can affect the ability of a battery to deliver a given amount of amps for a given period of time, the required battery capacity should not be determined solely on the basis of the Ah capacity that is required. Determining the required battery capacity really involves determining the specific type of cell from a given manufacturer that is capable of meeting all of the requirements of the duty cycle for which it is intended. A variety of methods can be used to determine the required battery capacity for a given application. The most common method of determining the required battery capacity for the following Saudi Aramco applications will be discussed in this section: • •

DC Systems UPS Systems

DC Systems

This section uses an example to explain a method that can be used to determine the required capacity of a battery for a dc system. The actual procedural steps and technical requirements for determining the required capacity of a battery for a dc system are located in Work Aid 2A. The Participant should refer to Work Aid 2A as necessary during this discussion. Before Work Aid 2A can be used to determine the required capacity of a battery for a dc system, the following aspects of the duty cycle diagram that were previously shown in Figure 14 must be defined: • •

Period Section



A period is an interval of time in the battery duty cycle during which the current is assumed to be constant for the purpose of cell sizing calculations. A section is an interval of time from the beginning of the duty cycle to the end of a period. A simple duty cycle that contains a single continuous load has one period and one section. A complex duty cycle has multiple periods and multiple sections. Figure 15 identifies the periods and the sections of the duty cycle diagram that were previously shown in Figure 14. Figure 15 shows that this particular duty cycle has five periods and five sections.



Periods and Sections of a Duty Cycle Diagram Figure 15



To determine the required battery capacity for a dc system, the following data concerning the installation must be obtained: •

The type (lead-acid or nickel-cadmium) of battery that is to be installed.

•

The nominal dc voltage of the system.

•

A list of all of the loads (including, for each load, the electrical ratings and the anticipated start and stop times) that are to be powered by the battery.

•

The lowest anticipated electrolyte temperature.

•

The battery manufacturer's technical literature.

The following data are used in the example capacity determination: •

The type of battery is lead-acid.

•

The nominal voltage is 120 vdc.

•

The lowest anticipated electrolyte temperature is 10 oC.

•

The typical manufacturer's technical literature from Work Aid 2A is used in the example.

•

The following loads are to be powered by the battery:

Load Type Continuous Momentary Noncontinuous Noncontinuous Momentary Random

Voltage Range 96-150 vdc 96-150 vdc 96-150 vdc 96-150 vdc 96-150 vdc 96-150 vdc

Current Draw 50 A 350 A 50 A 100 A 50 A 100 A

Start Time o min 0 min 1 min 30 min 179 min ---

Stop Time 180 min 1 min 90 min 90 min 180 min ---

Step 1 of the procedure that is in Work Aid 2A is to determine the minimum and the maximum allowable dc voltage for the example system. The nominal dc voltage of the example system is 120 vdc; therefore, from the table that is in the Technical Requirements section of Work Aid 2A, the minimum voltage for the example system is 105 vdc and the maximum voltage for the example system is 143 vdc.



Step 2 of the procedure that is in Work Aid 2A is to verify that the system loads are designed to operate within the minimum and the maximum voltage limits. Because the given data state that all of the loads are designed to operate on dc voltages that range from 96 to 150 vdc and the actual voltage range will be from 105 to 143 vdc, all of the loads can operate within the minimum and the maximum voltage limits of the example system. Step 3 of the procedure that is in Work Aid 2A is to determine the number of cells that are required in the example system. The nominal dc voltage of the example system is given as 120 vdc; therefore, from the table that is in the Technical Requirements section of Work Aid 2A, the required number of cells for the example system is 60 cells. Step 4 of the procedure that is in Work Aid 2A is to verify that the number of cells is correct for the application. The calculations for this verification were previously explained in the section titled "Battery Voltage". Step 5 of the procedure that is in Work Aid 2A is to tabulate the load information into a duty cycle diagram. An explanation of this step was previously provided in the section titled "Duty Cycle Diagram". The duty cycle diagram for the example system is the same as the duty cycle diagram that was previously shown in Figure 15. Step 6 of the procedure that is in Work Aid 2A is to use the duty cycle diagram and the Cell Sizing Work Sheet to determine the appropriate cell size for the example system. The Cell Sizing Work Sheet provides a convenient format for performing and recording the calculational data. The completed Cell Sizing Work Sheet for this example is shown in Figure 16. The remainder of this discussion covers the substeps that are involved in completing the Cell Sizing Work Sheet. Substep 6a of the procedure that is in Work Aid 2A is to fill in the information that is at the top of the work sheet. All of this information can be directly transferred from the given data except for the minimum cell voltage. The minimum cell voltage must be calculated through use of the information that was determined in step 1 and step 3 of the procedure. The minimum cell voltage is calculated as follows:





Example Cell Sizing Work Sheet Figure 16



Substep 6b of the procedure that is in Work Aid 2A is to fill in the load amperes that are required for each period of the duty cycle. The load amperes for each period of the duty cycle diagram (Figure 15) are recorded in the appropriate block of column (2) in the work sheet (Figure 16). For example, Figure 15 showed that the load for period 1 is 400 amperes; therefore, 400 is written in each of the blocks of column (2) of the work sheet that contains A1. The "A" designates amperes, and the "1" designates period 1. Because the duty cycle diagram only contains five sections, only the first five sections of the work sheet are used. Also, because the amperes of period 3 (A3) are greater than the amperes of period 2 (A2) and because the amperes of period 5 (A5) are greater than the amperes for period 4 (A4), only sections 1, 3, and 5 of the work sheet need to be filled out. Substep 6c of the procedure that is in Work Aid 2A is to fill in the duration (minutes) of each period of the duty cycle. The duration of each period of the duty cycle diagram (Figure 15) in minutes is recorded in the appropriate block of column (4) of the work sheet (Figure 16). For example, Figure 15 showed that the duration of period 1 is one minute; therefore, 1 is written in each of the block of column (4) of the work sheet that contains M1. The "M" designates minutes and the "1" designates period 1. Substep 6d of the procedure that is in Work Aid 2A is to perform the calculations that are indicated in column (3) of the work sheet (Figure 16). The results of the calculations should be recorded as positive or as negative values. As an example, the following are the calculations that are performed for section 3 of the work sheet: A1 - 0 400 - 0 400

= = =

Change in Load for Period 1 Change in Load for Period 1 Change in Load for Period 1

A2 - A1 100 - 400 -300

= = =


A3 - A2 200 - 100 100

= = =




Substep 6e of the procedure that is in Work Aid 2A is to perform the calculations that are indicated in column (5) of the work sheet (Figure 16). The times (T) that are being calculated are the times from the beginning of the period to the end of the section. As an example, the following are the calculations that are performed for section 3 of the work sheet: T T T

= = =

M1 + M2 + M3 1 + 29 + 60 90

T T T

= = =

M2 + M3 29 + 60 89

T T

= =

M3 60

Substep 6f of the procedure that is in Work Aid 2A is to record the battery capacity factor (R T or K T from the manufacturer's literature) in column (6) of the work sheet (Figure 16) for each of the discharge times that were calculated in substep 6e. For this example, the typical R T curves for a lead-acid battery that are located in the Technical Requirements section of Work Aid 2A are used to complete this substep. The typical R T curves are used as follows: •

The curve that corresponds to the time that is indicated in column (5) of the work sheet (Figure 16) must be located.

•

The time curve is followed to the left until it intersects the curve for the minimum cell voltage, which is the 1.75 volt curve for the example system.

•

An imaginary line that is perpendicular to the x-axis is then drawn through the point of intersection of the time curve and the minimum cell voltage curve such that it intersects the x-axis.

•

The point at which the imaginary line intersects the x-axis is the capacity factor (R T or amps per positive plate) for that discharge time.

Substep 6g of the procedure that is in Work Aid 2A is to calculate and record the cell size that is required for each period of the duty cycle as indicated in column (7) of the work sheet (Figure 16). The results of the calculation that are positive numbers should be recorded in the positive values column, and the results that are negative numbers should be recorded in the negative values column. As an example, the following are the calculations that are performed for section 3 of the work sheet:



(3) / (6) 400 / 59 6.78

= = =

# of Positive Plates Required # of Positive Plates Required # of Positive Plates Required

(3) / (6) -300 / 59 -5.08

= = =


(3) / (6) 100 / 73 1.37

= = =


Substep 6h of the procedure that is in Work Aid 2A is to calculate the algebraic totals and subtotals for each section of the duty cycle and to record the sums in column (7) of the work sheet (Figure 16). The results for each section are as follows: • • •

Section 1 total is 2.27 positive plates Section 2 total is 3.07 positive plates Section 3 total is 2.82 positive plates

Substep 6i of the procedure that is in Work Aid 2A is to record the maximum section size that is indicated in column (7) of the work sheet (Figure 16) on line (8) of the work sheet and to record the random section size on line (9) of the work sheet. These two values are then added together and their sum is recorded on lines (10) and (11) of the work sheet. Substep 6j of the procedure that is in Work Aid 2A is to select and record the appropriate cell size temperature correction factor on line (12) of the work sheet (Figure 16). The table of cell size temperature correction factors is located in the Technical Requirements section of Work Aid 2A. Because the lowest anticipated electrolyte temperature for the example system is 10 oC, the cell size temperature correction factor is 1.19. Substeps 6k and 6l of the procedure that is in Work Aid 2A are to respectively record the design margin and the aging factor on lines (13) and (14) of the work sheet (Figure 16). The design margin and the aging factor are stated in the Technical Requirements section of Work Aid 2A. The design margin is 110% and the aging factor is 125%. Substep 6m of the procedure that is in Work Aid 2A is to calculate the product of lines (11), (12), (13), and (14) and to enter this value on line (15) of the work sheet (Figure 16). This calculation is performed as follows:



Line (15) Line (15) Line (15)

= = =

(11) x (12) x (13) x (14) 3.64 x 1.19 x 1.10 x 1.25 5.96

Substep 6n of the procedure that is in Work Aid 2A is to round the fractional number of positive plates that are indicated on line (15) of the work sheet (Figure 16) up to the next higher integer. This value is then recorded on line (16) of the work sheet. The next higher integer for the example system is six. Substep 6o of the procedure that is in Work Aid 2A is to calculate the total number of plates that are required. This calculation is performed as follows: Total Number of Plates Total Number of Plates Total Number of Plates

= = =

2 (Number of Positive Plates) + 1 2 (6) + 1 13

Once the total number of plates is known, the manufacturer's typical standard cell data that are in the Technical Requirements section of Work Aid 2A can be used to determine the specific type of cell that is required for the example system. Because the example system requires a cell that has 13 plates, the type of TCCX-825 cell should be used. This cell is rated to deliver 825 Ah at the eight hour rate at an electrolyte temperature of 25 oC to a final cell voltage of 1.75 vdc. UPS System

Because the only load that should be connected to a UPS system is the UPS system inverter, determining the capacity of UPS system batteries is less involved than determining the capacity of dc system batteries that supply complex duty cycles. For the purpose of determining the required capacity of a UPS system battery, the UPS system is treated as a single continuous load with a duration that is equal to the specified battery protection period. The specified battery protection period for most Saudi Aramco UPS systems is 15 minutes or 30 minutes. The remainder of this section explains a method that can be used to determine the required capacity of a UPS system battery through use of an example. The actual procedural steps and technical requirements for determining the required capacity of UPS system batteries are located in Work Aid 2B. The Participant should refer to Work Aid 2B as necessary during this discussion.



To determine the required capacity of a UPS system battery, the following data concerning the installation must be obtained: • • • • • • • •

The The The The The The The The

type (lead-acid or nickel-cadmium) of battery that is to be installed. nominal ac load voltage and the number of phases. kVA rating of the UPS system. power factor of the critical ac load. efficiency of the inverter. design battery protection period. lowest anticipated electrolyte temperature. battery manufacturer's technical literature.

The following data are used in the example capacity determination: • • • • • • • •

The type of battery is nickel-cadmium. The nominal ac load voltage is 120 vac, single phase. The kVA rating of the UPS system is 30 kVA. The power factor of the critical ac load is .90. The efficiency of the inverter is 80%. The design battery protection period is 15 minutes. The lowest anticipated electrolyte temperature is 25 oC. The typical manufacturer's technical literature from Work Aid 2B is used in the example.

Step 1 of the procedure that is in Work Aid 2B is to determine the minimum battery voltage and the number of cells that are required based on the nominal ac load voltage. The nominal ac load voltage is 120 vac; therefore, from the table that is in the Technical Requirements section of Work Aid 2B, the minimum battery voltage for the example system is 105 vdc and the number of cells that are required for the example system is 92. Step 2 of the procedure that is in Work Aid 2B is to calculate the required battery kW for the example system, using the formula that is provided in the Technical Requirements section of Work Aid 2B. This calculation is performed as follows: Step 3 of the procedure that is in Work Aid 2B is to calculate the minimum required battery current for the example system, using the formula that is provided in the Technical Requirements section of Work Aid 2B. This calculation is performed as follows:



Step 4 of the procedure that is in Work Aid 2B is to multiply the battery current that was calculated in step 3 by the applicable cell size temperature correction factor, the design margin, and the aging factor, which are located in the Technical Requirements section of Work Aid 2B. The product is calculated as follows: Actual Battery Current Actual Battery Current

= =

321 amps x 1 x 110% x 125% 441 amps

Step 5 of the procedure that is in Work Aid 2B is to use the battery manufacturer's standard cell capacity table, the design protection period (15 minutes for the example system), and the actual battery current that was calculated in step 4 to determine the specific type of cell that is needed for this application. The typical standard cell capacity table that is provided in the Technical Requirements section of Work Aid 2B is used to complete this step. The following is an explanation of how to use the standard cell capacity table to determine the specific type of cell that is needed. •

The time column that corresponds to the design protection period of the system must be located at the top of the table. If the design protection time falls between two of the standard periods, the higher standard time column is used.

•

This time column is followed down to the row that contains the required battery current. If the required battery current falls between two of the standard currents, the higher standard current is used.

•

The row that contains the required current is followed to the far left column of the table. The far left column of the table shows the specific type of cell that is needed for the system.

The specific type of cell that is needed for the example system is type SBH 230. This cell is rated to deliver 449 amps for 15 minutes at an electrolyte temperature of 25 oC to a final cell voltage of 1.10 vdc.



DETERMINING BA TTERY CHARGER SIZE FOR USE IN TYPICAL SAUDI ARAMCO APPLICATIONS A properly sized battery charger is essential in order for a dc system or a UPS system to properly operate. This section of the Module provides information on the following topics that are pertinent to determining the battery charger size for typical Saudi Aramco applications: • • • • •

Types Current and Voltage Ratings Operational Requirements Environmental Ratings Sizing

Types The battery chargers for Saudi Aramco dc/UPS systems should be constant-potential, semiconductor, static-type chargers that are designed to simultaneously supply dc power to a floattype battery and to the connected dc loads. The following general designs are acceptable: • •

Static Plate Rectifier Solid State Rectifier

Static Plat e Rectifier

Figure 17 shows a single-unit static plate rectifier. The static plate rectifier can contain selenium discs or a group of copper or aluminum discs that contain oxidized selenium or silicon on one side. The discs are clamped together and have electrical connections on both sides. The discs are supported on an insulated shaft that is secured by insulator clamps at each end. The static plate rectifier is mounted in the battery charger housing through use of a mounting clasp that is connected at the ends of the static plate rectifier. The static plate rectifier converts an ac input to a dc output. The static plate rectifier accomplishes this conversion by allowing current to flow only in one direction. When an ac voltage is applied to the static plate rectifier, current can flow from the oxidized side of the disc to the adjacent copper or aluminum side but cannot flow in the opposite direction. This rectification principle is known as half-wave rectification.



Static Plate Rectifier Figure 17

Figure 18 shows two possible ways that static plate rectifiers can be configured to produce a dc output. Figure 18A is a half-wave rectifier circuit. In this circuit, an ac voltage (E AC) is applied to the circuit through the transformer secondary winding. The static plate rectifier only allows current to flow in one direction through the load. The voltage that is applied to the load will only occur during one half of the applied ac sinewave; therefore, the shape of the resultant E DC is a series of half peaks and flat spots. Figure 18B shows a full-wave rectifier circuit. The full-wave rectifier also receives ac power from a transformer secondary winding; however, two static plate rectifiers are connected in this circuit. The load is connected between the static plate rectifiers and a center tap of the transformer. The two static plate rectifiers are electrically connected so that one rectifier conducts on the positive half-cycle of the ac supply voltage and so that the other rectifier conducts on the negative half of the ac supply voltage. The output (E DC) to the dc load is a full-wave voltage.



Static Plate Rectifier Operation Figure 18

The static plate rectifier replaced the old motor-generator systems. Because a static plate rectifier has no moving parts, it is smaller and requires less maintenance than the motorgenerator systems. One disadvantage of the static plate rectifier is the heat that is generated from the flow of current through the resistance of the oxide layer. This heat decreases the efficiency and increases the cost of operating static plate rectifiers.



Solid Sta te R ectifier

The solid state rectifier type of battery charger uses power diodes and/or silicon controlled rectifiers (SCR's) to convert ac to dc. Figure 19 shows a simplified diagram of a solid state rectifier. The operation of the solid state rectifier is very similar to the operation of the static plate rectifier. The diodes that make up the solid state rectifier only allow current flow in one direction. The diodes are arranged to provide a path for current flow during each half cycle of the input ac sinewave. The input ac sinewave is supplied to the solid state rectifier from the multi-tap transformer secondary. Multi-tap transformer secondaries permit a coarse mechanical adjustment of the voltage that is supplied to the dc load. During the positive half of the ac sinewave, diodes D1 and D4 conduct, which allows current flow to the system battery from negative to positive. During the negative half of the ac sinewave, diodes D2 and D3 conduct, which also allows current flow to the system battery from negative to positive. The diode pairs alternately conduct as long as an ac sinewave is applied, which maintains dc current flow to the system battery from negative to positive. The resultant output wave shape that is supplied to the system battery is also shown in Figure 19. The major advantages of the solid state rectifier are that it is smaller and more efficient than the static plate rectifier. The main disadvantage of the solid state rectifier is that it costs more than the static plate rectifier. Current and Voltage Ratings The standard continuous-duty direct current output ratings of Saudi Aramco battery chargers should be one of the following values: 1A, 3A, 6A, 12A, 15A, 20A, 25A, 35A, 50A, 75A, 100A, 150A, 200A Battery chargers that are larger than 200A should be rated in 100A increments. The ac input voltage ratings are based on the available ac voltage and the required system output voltage. The following are the nominal ac input voltages for Saudi Aramco battery chargers:

• •

120 vac 240 vac

• •

208 vac 480 vac



Solid State Rectifier Figure 19



The output voltage ratings of a battery charger are dictated by the type of battery that the charger is intended to charge. The output voltage ratings of the battery charger are based on the float, equalizing, and final volts per cell of the battery to which it is connected. The maximum voltage output of the battery charger is equal to the following: Battery charger maximum output voltage = Equalizing voltage per cell x number of cells Figure 20 shows the typical volts per cell ratings for different types of batteries. The float, equalizing, and final volts per cell ratings are multiplied by the number of cells that are in the battery to determine the necessary battery charger output voltage rating.

Typical Volts Per Cell Ratings for Different Battery Types Figure 20



Operational Requirements The operational requirements for Saudi Aramco battery chargers fall into the following categories: • • •

Output Regulation Controls Filters

Output Regulation

The voltage output of a battery charger must be regulated to ensure that the proper charge voltages are applied to the battery. Charge voltages that are outside of the tolerances of the battery could cause excessive gassing in the cells. The exact amount of output voltage control that is required depends on the rated size of the battery charger and the type of charge that is being performed. The following are the output voltage regulation requirements for Saudi Aramco battery chargers: Battery Charger Size < 10 kW > 10 kW

Float Operation Mode +/- 1% +/- 1%

Recharge (Equalize) Operation Mode +/- 2% +/- 1%

Battery charger output regulation also refers to the battery charger's ability to maintain the rated output during operation of the battery charger. A battery charger must be able to maintain nominal rated voltage output during load changes, battery removal, and variations in the input voltage. The battery charger output voltage must return to within the previously mentioned limits within two seconds when the load is changed or when the battery is removed, and no voltage variation should result in exceeding the rated output voltage. The battery charger also must maintain an output voltage that is within the specified voltage tolerances during variations in the battery charger input voltage. Controls

Separate and independent controls are required for the adjustment of the float and recharge or equalizing voltage. The battery charger should provide the following ranges of float voltage output control at its nominal ac input voltage and half load:



Adjustment Range Per Cell Lead-Acid Cells 2.15 - 2.25 VPC Nickel-Cadmium 1.40 - 1.47 VPC The battery charger should also provide the following ranges of equalizing voltage control at its nominal ac input voltage and half load: Adjustment Range Per Cell Lead-Acid Cells 2.25 - 2.40 VPC Nickel-Cadmium 1.50 - 1.65 VPC The following formula is used to calculate the required float and equalizing voltage control ranges for battery chargers: Charger Output Voltage

=

VPC x Number of Cells

For example, the required float and equalizing voltage control range for a 60-cell lead-acid battery are calculated as follows: Float Voltage Adjustment Range: Minimum Charger Output Voltage

Maximum Charger Output Voltage

= = = = = =

VPC x Number of Cells 2.15V x 60 129V VPC x Number of Cells 2.25V x 60 135V

= = = = = =

VPC x Number of Cells 2.25V x 60 135V VPC x Number of Cells 2.40V x 60 144V

Equalizing Voltage Adjustment Range: Minimum Charger Output Voltage

Maximum Charger Output Voltage



The battery charger must also have an input circuit breaker and an output circuit breaker or disconnect switch. The input circuit breaker should have an interrupting capacity that is equal to or greater than the available short-circuit current. The minimum capacity is 14,000 symmetrical rms amps. Current-limiting fuses may also be provided to meet the short-circuit current requirements. The output circuit breaker or disconnect switch must be able to simultaneously disconnect both output lines. Supervisor y Contr ols

The battery charger should be equipped with the following supervisory controls: • • • • • • •

End of Discharge Relay Ground Detection Relay Equalizing Timer Low Voltage Relay Overvoltage Relay Charger Failure Relay Low Current Relay

End of Discharge Relay - The end of discharge relay senses the battery bus voltage and

actuates when the battery has discharged to a predetermined level. The end of discharge relay should activate when the battery voltage equals 1.75 vpc for lead-acid batteries and 1.10 vpc for nickel-cadmium batteries. The setpoint of the relay should be adjustable from +5% to -5%. Operation of this relay indicates that the battery needs to be charged. Gr ound Detection Relay - The ground detection relay senses ground faults on the positive

or the negative battery bus. Operation of this relay indicates the presence of a ground fault. Equa lizing Timer - The equalizing timer allows unsupervised control of the equalizing

charge. The timer is manually set to the desired equalizing time period. When the battery charger is placed in the equalize charge mode, the timer automatically returns the battery charger to the float charge mode after the preset time has elapsed. Low Voltage Relay - The low voltage relay provides an indication that the dc output

voltage has dropped below a preset value. This relay must be adjustable to allow for field settings of the minimum battery output voltage.



Overvoltage Relay - The overvoltage relay provides an indication that the dc output

voltage has increased above a preset value. The relay must be adjustable to allow for field setting of the maximum battery output voltage. Char ger Failure Relay - The charger failure relay provides an indication of a loss of the

charger's dc output. The battery charger failure relay actuates on a charger output failure regardless of the cause. The charger failure relay is intended to alert the operator that the charger is no longer in operation, but it does not provide an exact indication as to the cause of charger output failure. Low Curr ent Relay - The low current relay provides an indication that the dc output

current of the battery charger has dropped to a value that is less than two percent of the rated output. Filters

Filters are placed on the output of a battery charger to reduce the ac ripple that is in the dc power that is supplied to the system load. The filter requirements for Saudi Aramco battery chargers vary dependent on the size of the battery charger and the type of dc load. The ac ripple voltage of battery chargers that are rated for 10 kW or less should be filtered to a maximum of 30 mV rms or 100 mV rms dependent on the requirements of the connected load. The ac ripple voltage of battery chargers that are rated for more than 10 kW should be filtered to 100 mV rms. All battery chargers that supply UPS systems also should be filtered at the 100 mV rms level. The battery chargers that supply dc systems that are dedicated to only emergency lighting systems do not require filters. Environmental Rat ings Battery chargers normally are operated inside of buildings that are air-conditioned to 25 oC (77 oF). However, in the event of a loss of air conditioning, the battery chargers must be rated to operate for a period of eight hours in the following environment: • • •

Ambient temperatures that range from 0 oC (32oF) to 50oC (122 oF). Relative humidity up to 100%. Sand, salt, and dust-laden atmosphere.



Sizing Proper sizing of a battery charger is important to help ensure that the dc/UPS system in which the charger is installed operates properly over its designed life. Proper sizing of a battery charger requires consideration of the following: • • •

Load Criteria Voltage Rating Calculate Current Rating

Load Criteria

The magnitude of the system load that is supplied from the battery charger must be considered in order to determine the proper size of the battery charger. The type of load, whether a dc system or a UPS system, must also be considered when sizing the battery charger. A battery charger in a dc system can supply many different loads at the same time. The battery charger of a dc system must be large enough to provide power to all of the loads and must still be able to charge the battery. A battery charger in a UPS system must be large enough to operate the inverter and still be able to charge the battery. The battery charger and battery of a UPS system should be dedicated to the operation of the UPS inverter; the battery and charger should not supply any other dc loads. If a particular UPS installation has necessary dc loads that cannot be supplied from a separate battery, these loads can be connected to the UPS battery provided that the loads do not exceed 10% of the battery's total Ah capacity. Voltage Ra ting

Determination of the required battery charger output voltage is accomplished by multiplication of the equalizing voltage and the float voltage by the number of cells that are in the battery. The exact equalizing voltage and float voltage that are required are determined by the battery manufacturer. For example, the required float voltage and equalizing voltage ratings of a battery charger for a battery with 60 cells, a float voltage of 2.2 vpc, and an equalize voltage of 2.33 vpc is determined as follows: Float Voltage Rating

= =

60 cells x 2.2 vpc 132 volts



Equalizing Voltage Ratings

= =

60 cells x 2.33 vpc 140 volts

Calculate Cur rent Ra ting

The method that is used to calculate the current rating of battery chargers for dc systems is different than the method that is used to calculate the current rating of battery chargers for UPS systems. This sections explain both methods. DC Systems - The current output rating of a battery charger for a dc system is calculated

through use of the following equation: where:

A

=

Ampere rating of charger

S.F.

=

Service factor: 1.1

L

=

DC system continuous load

BIF

=

Battery inefficiency factor: 1.15 for lead-acid batteries 1.4 for nickel-cadmium batteries

Ah

=

Ampere-hours removed during a discharge period

RT

=

Recharge time

K(t)

=

Temperature compensation: 0.83 for operation at 41 to 50 degrees C and 1.0 for operation at or below 40

and

degrees C As an example, it is assumed that a dc system has the following parameters: • •

Battery - 60 lead-acid cells Load - 100 ampere-continuous

• • •

Discharge cycle - 300 Ah Recharge time - 8h Battery Room Temperature - 40 oC (104oF)

Through use of the assumed dc system parameters, the battery charger current rating is calculated as follows:



In this example, the battery charger requires a minimum output current rating of 153 amperes to simultaneously supply the load and charge the battery. Because the calculation indicates a non-standard output current rating (153 amperes), the 200 ampere standard size unit should be used. UPS System - The current output rating of a battery charger for a UPS system is based

on the kilowatts (kW) that are required for the inverter. The following equations are used to calculate the current rating of battery chargers for UPS systems: where:

kW PF

= =

Inverter Load Power Factor

where:

E(L)

=

Minimum input battery voltage (also the battery final discharge voltage)



Sizing (Cont'd) As an example, assume that a UPS system has the following parameters: • • • • • • • •

Battery - 60 lead-acid cells Battery Minimum Voltage - 1.75 VPC Inverter Load - 30.0 kVA Power Factor - 0.80 Discharge Cycle - 200 Ah Recharge Time - 8h Efficiency - 0.70 Temperature - 40 oC

Through use of the assumed UPS system parameters, the battery charger current rating is calculated as follows: In this example, the battery charger requires a minimum output current rating of 356 amperes to simultaneously supply the inverter and charge the battery. Because the calculation indicates a non-standard output current rating (356A), the 400 ampere standard unit should be used.



WORK AID 1: PROCEDURE AND TECHNICAL A ND ECONOMIC REQUIREMENTS FROM SADP-P-103 AND ESTABLISHED ENGINEERING PRACTICES FOR DETERMINING THE TYPE OF BATTERY FOR USE IN TYPICAL SAUDI ARAMCO APPLICATIONS Wor k Aid 1A:

Pr ocedur e

1.

Review the cyclic performance of each type of battery. Determine which battery types can meet the needs of the installation.

2.

Review the effects of temperature on each type of battery. Determine which battery types can successfully operate in the expected ambient temperature of the installation.

3.

Review the maintenance requirements of each type of battery. Determine which battery types are suitable for the installation based on the anticipated manpower and logistics of the installation.

4.

Review the service life of each type of battery. Determine which battery types can meet the planned life of the installation.

5.

Review the relative cost of each type of battery. Select the lowest cost battery type that meets all of the technical requirements of the installation.

W or k Aid 1B:

T ech nica l a nd E con om ic Req uir em en ts

Cyclic Performance: Plante Lead-Acid Lead Calcium Lead Antimony Nickel-Cadmium

-

Will withstand frequent cycling. Will not withstand frequent cycling. Will withstand frequent cycling. Will withstand frequent cycling.



Effects of Temperature: All lead acid batteries suffer a 50% reduction in service life for each 10 oC temperature rise above 25 oC. Nickel-cadmium batteries suffer little or no loss in service life due to elevated temperatures. Maintenance Requirements: Plante Lead-Acid Lead Calcium Lead Antimony Nickel-Cadmium

-

Medium maintenance required. Medium maintenance required. High maintenance required. Low maintenance required.

-

20 to 25 years 12 to 15 years 10 to 12 years 15 to 25 years

-

High capital cost Medium capital cost Lowest capital cost Highest capital cost

Service Life Plante Lead-Acid Lead Calcium Lead Antimony Nickel-Cadmium Relative Capital Cost Plante Lead-Acid Lead Calcium Lead Antimony Nickel-Cadmium



WORK AID 2: PROCEDURE AND TECHNICAL REQUIREMENTS FROM SADPP-103 AND ESTAB LISHED ENGINEERING PRACTICES FOR DETERMINING THE SIZE OF BA TTERY FOR USE IN TYPICAL SAUDI ARAMCO A PPLICATIONS Work Aid 2A:

Procedure and Technical Requirements for Determining the Appropriate Size of Battery for Use in a Typical Saudi Aramco DCS

Procedure

1.

Determine the minimum and the maximum allowable dc voltages for the system, using the table that is provided in the Technical Requirements section of this Work Aid.

2.

Verify that all of the system loads are designed to operate within the minimum and the maximum voltage limits.

3.

Determine the number of cells that are required, using the table that is provided in the Technical Requirements section of this Work Aid.

4.

Verify that the number of cells is correct for the application, using the formulas that are provided in the Technical Requirements section of this Work Aid.

5.

Tabulate the load information into a duty cycle diagram.

6.

Use the duty cycle diagram and the Cell Sizing Work Sheet that is shown in Figure 23 to determine the appropriate cell size for the application.

6a.

Fill in the necessary information that is at the top of the Cell Sizing Work Sheet.

6b.

Fill in the amperes that are required for each period of the duty cycle in column (2) of the work sheet, using the information from the duty cycle diagram that was tabulated in step 5.

6c.

Fill in the duration (minutes) of each period of the duty cycle in column (4) of the work sheet, using the information from the duty cycle diagram that was tabulated in step 5.

6d.

Calculate and record the changes in amperes as indicated in column (3) of the work sheet. Also record whether the changes are positive or negative.



6e.

Calculate and record the times from the start of each period of the duty cycle to the end of the section of the duty cycle as indicated in column (5) of the work sheet.

6f.

Record the cell capacity factor (R T or K T from the manufacturer's literature) in column (6) of the work sheet for each discharge time that was calculated in column (5). Note: Most battery manufacturers provide curves that show R T (amps per positive plate) rather than K T (ratio of rated ampere-hour capacity to the amperes that can be supplied for a given number of minutes). Also, the curves are different for each type of cell that a given manufacturer produces. The actual manufacturer's curves should be obtained to perform an actual size determination; however, for the purposes of this Exercise, use the typical curves that are provided in Figure 24 in the Technical Requirements section of this Work Aid.

6g.

Calculate and record the cell size that is required for each period of the duty cycle in column (7) of the work sheet. Note that a separate column is provided for positive and negative values.

6h.

Calculate and record the algebraic subtotals and totals for each section of the duty cycle in column (7) of the work sheet.

6i.

Record the maximum section size (i.e., the largest total) from column (7) on line (8), record the random section size on line (9), and record the uncorrected size (US) on lines (10) and (11).

6j.

Select the appropriate cell size temperature correction factor from the Technical Requirements section of this Work Aid and record this value on line (12) of the work sheet.

6k.

Record the design margin from the Technical Requirements section of this Work Aid on line (13) of the work sheet.

6l.

Record the aging factor from the Technical Requirements section of this Work Aid on line (14) of the work sheet.

6m.

Calculate the product of lines (11), (12), (13), and (14), and enter this value on line (15) of the work sheet.



6n.

If the capacity factor R T was used to make the calculations in the work sheet, the value that is on line (15) indicates the number of positive plates that are required for this application. When line (15) indicates a fractional number of positive plates, the value should be rounded up to the next higher integer and this rounded value should be recorded on line (16) of the work sheet. If the capacity factor K T was used to make the calculations in the work sheet, the value that is on line (15) indicates the required cell size in ampere-hours. If the number of ampere-hours that are indicated on line (15) does not match the capacity of one of the manufacturer's standard cell sizes, the next larger standard size cell is required. Note: The battery manufacturer's literature contains tables that show all of the standard cells that the manufacturer produces and the capacity of each cell. In the case of lead-acid cells, the tables also show the number of plates that are in each of the standard cells. The actual manufacturer's tables should be obtained to perform an actual size determination; however, for the purposes of this Exercise, use the typical table that is shown in Figure 25 in the Technical Requirements section of this Work Aid.

6o.

If the value that is on line (16) indicates the number of positive plates that are required, use this value to calculate the total number of plates that are required (Total Number of Plates = 2[Number of Positive Plates] + 1). Use the actual number of plates that are required and the manufacturer's table to determine the specific type of cell that is required for this application. Record this type on line (17) of the work sheet. If the value that is on line (16) indicates the ampere-hour capacity that is required, use this value and the manufacturer's table to determine the specific type of cell that is required for this application. Record this type on line (17) of the work sheet.





Cell Sizing Work Sheet Figure 23



Technical Requir ements

Table of Minimum and Maximum Allowable dc System Voltages:

Nominal System Voltage 12 vdc 24 vdc 48 vdc 120/125 vdc 240/250 vdc 360/375 vdc

Minimum System Voltage 10.5 vdc 21.0 vdc 42.0 vdc 105 vdc 210 vdc 315 vdc

Maximum System Voltage 15.5 vdc 30.0 vdc 58 vdc 143 vdc 286 vdc 429 vdc

Table of Required Number of Cells:

Nominal Battery System Voltage (vdc) 12 24 48 120/125 240/250 360/375

Number of Cells

Lead-Acid 6 12 24 60 120 180

Nickel-Cadmium 10 19 37 92 184 276



Formulas for Calculating the Minimum and Maximum Number of Cells:

Typical Manufacturer's R T Curves:

Typical R T Curves for a Lead-Acid Battery Figure 24



Table of Cell Size Temperature Correction Factors: Electrolyte Temperature C (F) -1.1 4.4 10 18.3 25

Cell Size Correction Factor

(30) (40) (50) (65) (77)

Lead-Acid 1.43 1.30 1.19 1.08 1.00

Nickel-Cadmium 1.30 1.20 1.14 1.02 1.00

Design Margin: To compensate for load growth, a 110% design margin should be included in the size of all Saudi Aramco batteries. Aging Factor: To compensate for the anticipated normal capacity loss over the life of a battery installation, a 125% aging factor should be included in the size of all Saudi Aramco batteries. Typical Table of Battery Manufacturer's Standard Cell Data:

Typical Standard Cell Data Figure 25



Work Aid 2B:

Procedure and Technical Requirements for Determining the Appr opr iate Size of Bat ter y for Use in a T ypical Saud i Ar amco UPS System

Procedure

1.

Determine the minimum battery voltage and the number of cells that are required based on the nominal ac load voltage, using the table that is provided in the Technical Requirements section of this Work Aid.

2.

Calculate the required battery kW, using the formula that is in the Technical Requirements section of this Work Aid.

3.

Calculate the minimum required battery current, using the formula that is provided in the Technical Requirements section of this Work Aid.

4.

Multiply the battery current that was calculated in step 3 by the applicable temperature correction factor, the design margin, and the aging factor that are provided in the Technical Requirements section of this Work Aid. This product is the actual amount of current that is required.

5.

Use the manufacturer's standard cell capacity tables, the design protection period for the installation, and the current that was calculated in step 4 to determine the specific type of cell that is needed for this application. Note: The battery manufacturer's literature contains tables that show all of the standard cells that the manufacturer produces and the capacity of each cell. The actual manufacturer's tables should be obtained to perform an actual size determination; however, for the purposes of this Exercise, use the typical table that is shown in Figure 26 in the Technical Requirements section of this Work Aid.



Technical Requir ements

Battery Voltages and Required Number of Cells for UPS Systems: AC Load Voltage VAC 120 240 120/208

Number Phases 1_ 1_ 3_

Circuit Wires 2 3 4

Min Battery Voltage 105 210 182

Max Battery Voltage 143 286 248

Lead-Acid No. Cells 60 120 90

Nickel-Cadmium No. Cells 92 184 160

Formula for Calculating Battery kW:

Formula for Calculating Battery Current:

Table of Cell Size Temperature Correction Factors: Electrolyte Temperature C (F) -1.1 4.4 10 18.3 25

Cell Size Correction Factor

(30) (40) (50) (65) (77)

Lead-Acid 1.43 1.30 1.19 1.08 1.00

Nickel-Cadmium 1.30 1.20 1.14 1.02 1.00



Design Margin: To compensate for load growth, a 110% design margin should be included in the size of all Saudi Aramco batteries. Aging Factor: To compensate for the anticipated normal capacity loss over the life of a battery installation, a 125% aging factor should be included in the size of all Saudi Aramco batteries. Typical Table of Battery Manufacturer's Standard Cell Data:



Figure 26. Typical Stand ar d Cell Cap acity Table



WORK AID 3: PROCEDURE, TECHNICAL REQUIREMENTS, AND FORMULAS FROM SADP-P-103 AND ESTABLISHED ENGINEERING PRACTICES FOR DETERMINING THE SIZE OF A BA TTERY CHARGER FOR USE IN TYPICAL SAUDI ARAMCO APPLICATIONS Wor k Aid 3A:

Pr ocedur e

Use this procedure to determine the appropriate size of a battery charger. 1.

Determine the required float voltage output rating.

2.

Determine the required equalizing voltage output rating.

3.

Determine the required current output rating.

Work Aid 3B:

Technical Requirements and Formulas for Determining Battery Char ger Voltage Ratings

Volts per cell requirements:

Battery Type Plante Lead-Acid Lead Antimony Lead Calcium Nickel-Cadmium

Float Volts Per Cell 2.15 to 2.25 2.15 to 2.25 2.15 to 2.25 1.40 to 1.47

Equalize Volts Per Cell 2.25 - 2.40 2.25 - 2.40 2.25 - 2.40 1.50 - 1.65

Final Volts Per Cell 1.75 1.75 1.75 1.10

Float Voltage Output Rating: Float Voltage = Number of Cells x Required Float Volts Per Cell Equalizing Voltage Output Rating: Equalize Voltage = Number of Cells x Required Equalize Volts Per Cell



Work Aid 3C:

Technical Requirements and Formula for Determining Battery Charger Current Ratings for dc Systems

Service Factor (S.F.): The service factor is 1.1 Battery Inefficiency Factor: 1.15 for lead-acid batteries 1.40 for nickel-cadmium batteries Nominal Battery Recharge Time for Different Applications: Battery Applications Essential Loads Manned Substations Unattended Substations Uninterrupted Power Systems

Recharge Hours 8 16 16 8

Temperature Compensation Factor: 0.83 for operation between 41 oC and 50 oC 1.0 for operation at or below 40 oC

where :

S.F. BIF Ah RT K(t)

= = = = =

Service factor Battery inefficiency factor Ampere-hours removed during discharge Recharge time Temperature compensation



Technical Requir ements and For mulas for Deter mining Battery Char ger C ur r ent Ra tings for UPS Systems Battery Inefficiency Factor: 1.15 for lead-acid batteries 1.40 for nickel-cadmium batteries Nominal Battery Recharge Time for Different Applications: Battery Applications Essential Loads Manned Substations Unattended Substations Uninterrupted Power Systems

Recharge Hours 8 16 16 8

Temperature Compensation Factor: 0.83 for operation between 41 oC and 50 oC 1.0 for operation at or below 40 oC where:

PF E(L) BIF Ah

= = = =

RT Kt

= =

Power factor Minimum battery voltage Battery inefficiency factor Ampere-hours removed during discharge period Recharge time Temperature compensation factor

UPS 2

Recommend Documents