IEEE Guide for Selection of Valve- Regulated Lead-Acid (VRLA) Batteries for Stationary Applications
1.Scope This guide describes describes methods methods for selectin selecting g the appropriat appropriatee type of valve-re valve-regulat gulated, ed, immobili immobilizedzed-elec electrol trolyte, yte, recomb recombina inant nt lead-a lead-acid cid batter battery y for any of a variet variety y of potent potential ial statio stationar nary y float float applic applicati ations ons.. Instal Installat lation ion,, maintenance, sizing, and consideration of battery types other than valve-regulated lead-acid batteries, are beyond the scope of this guide. Design of the dc system and sizing of the dc battery charger(s) are also beyond the scope of this guide.
2 .Re f e r e nc es This guide shall be used in conjunction with the following publications: IEEE Std 100-1992, The New IEEE Standard Dictionary of Electrical and Electronics Terms (ANSI). 1 IEEE Std 485-1997 485-1997,, IEEE Recommended Practice for Sizing Large Lead Storage Batteries for Generating Stations and Substations (ANSI). IEEE Std 1187-1996, IEEE Recommended Practice for Installation Design and Installation of Valve- Regulated Lead-Acid Batteries for Stationary Applications. IEEE IEEE Std 1188-1 1188-1996 996,, IEEE IEEE Recomm Recommend ended ed Practi Practice ce for Mainte Maintenan nance, ce, Testin Testing, g, and Replac Replaceme ement nt of ValveValveRegulated Lead-Acid (VRLA) Batteries for Stationary Applications.
3 .Defini t i ons The following definitions apply specifically to this guide. For other definitions, see IEEE Std 100-1992. Electroly olyte te in a VRLA VRLA cell cell that that has been been immobi immobiliz lized ed in absorb absorbent ent absorbed absorbed electroly electrolyte: te: Electr 5 . 1 separators. gelled electrolyte: Electrolyte electrolyte: Electrolyte in a VRLA cell that has been immobilized by the addition of a 5 . 2 gelling agent. immobilized electrolyte: Electrolyte electrolyte: Electrolyte in a VRLA cell that is retained by using either gelled or 5 . 3 absorbed electrolyte technology. oxygen recombination: recombination: The process by which oxygen is generated at the positive plates and 5 . 4 ultimately recombined with hydrogen ions at the negative plates and converted back to water. In this process, hydrogen gas formation and evolution are suppressed. (See annex A for more details.) oxygen recombination efficiency: The efficiency: The amount of oxygen ultimately converted to water at the 5 . 5 negative plates expressed as a percentage of the total amount of oxygen produced at the positive plates:
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O2 eff
=
O2 converted to water at the negative plates total O2 produced at the positive plates
× 100
valve-regulated lead-acid (VRLA) cell: A cell that is sealed with the exception of a valve that 5 . 6 opens to the atmosphere when the internal gas pressure in the cell exceeds atmospheric pressure by a preselected amount. VRLA cells provide a means for recombination of internally generated oxygen and the suppression of hydrogen gas evolution to limit water consumption. vented cell: A cell in which the products of electrolysis and evaporation are allowed to escape to 5 . 7 the atmosphere as they are generated. These cells are commonly referred to as “flooded.” 5 . 8
VRLA cell: See: valve-regulated lead-acid (VRLA) cell.
4 .T e chnol ogyov e r v i e w 5 . 1
Wa t e rl os s
Water in a vented lead-acid cell is lost during overcharge by a process known as electrolysis. In this process, water is converted to oxygen at the positive plates and to hydrogen at the negative plates. The oxygen and hydrogen gases are allowed to vent out of the cell into the atmosphere, resulting in the loss of water. In VRLA cells, the oxygen recombination cycle limits the water loss.
5. 2
Oxygenr ecombi nat i on
In a perfect water-to-oxygen-to-water cycle, all the oxygen produced at the positive plates on float or overcharge would be transported to the negative plates and converted back to water, with no water being lost from the cell. This is the basis for VRLA cell technology (see annex A for more details).
5 . 3
Ox y ge nt r a ns por tbe t we enpos i t i v ea ndne ga t i v epl a t e s
The efficiency of the oxygen recombination cycle depends primarily on the ability to transport the oxygen generated at the positive plates to the negative plates. In vented lead-acid cells, the transport process is impeded by the bulk liquid electrolyte, and oxygen is liberated to the atmosphere. The migration of oxygen through liquid sulfuric acid electrolyte is approximately 10 000 times slower than it is through air [B1], [B2], [B3] 2. VRLA technology provides voids (gas passages) between positive and negative plates through which oxygen transport is greatly enhanced.
5 . 4
Ge l l e de l e ct r ol y t et e chnol ogy
Gelled electrolyte cells are designed such that voids develop in the gel. These voids serve as passages through which oxygen transport to the negative plates is enhanced.
5 . 5
Abs or be de l e ct r ol y t et e chnol ogy
Absorbed electrolyte cells are designed with an absorbent separator that is approximately 95% filled with liquid electrolyte. The remaining voids provide for optimized oxygen transport from positive to negative plates.
5 . 6
Hy dr oge ne v ol ut i on
It is possible to design VRLA cells in which, under normal float conditions, the oxygen recombination will operate at virtually 100% efficiency. However, even under normal float conditions, some water will be lost by electrolysis. There are reactions that occur at the positive plates whose only possible corresponding reaction at the negative plate is the formation of hydrogen gas. The most familiar of these reactions is corrosion of the lead or lead alloy positive grid to lead dioxide, which results in hydrogen evolution at the negative plates. These reactions cannot be prevented. (See annex A for more details.)
5 . 7
Pr e ss ur er e gul a t i onv al v e
The internal cell pressure caused by the evolved gases is regulated by a valve that allows them to escape periodically. This is the origin of the term valve regulated . VRLA valves operate over a relatively narrow range, typically within the limits of 0.5–50 psig, depending upon design, allowing the escape of unrecombined gases and preventing the backflow of air into the cell.
5 . 8
Cat al y st s
2The numbers in brackets preceded by the letter B correspond to those of the bibliography in annex B.
The following text was adapted from a Jim McDowall presentation:
Some battery manufacturers use catalysts in the head space mainly to reduce water loss and corrosion. This is a relatively new technology, and time will show how much value it has. Catalysts probably help overcome problems with negative self-discharge (if they exist), bring the positives to a more favorable polarization level, and reduce grid corrosion in normal operation (which will reduce water loss from grid corrosion. Catalysts cannot completely eliminate water loss (although they may help reduce it), nor can they eliminate hydrogen evolution (although they may reduce the amount of hydrogen lost to the air under normal operation of the cell). For those choosing to pay a small premium for the potential promises of this technology, they need to be aware that catalysts will affect the start and stop points and slope of temperature compensation (contact the battery manufacturer for recommended values). The effect of cell aging on polarization, and the potential for poisoning the cell are among the potential issues surrounding the catalyst technology which have not been fully explored.
4 .Compa r i s onso fv e nt e d( floode d)a ndv al v er e gul a t e dt e chnol ogi e s 5 . 1
Re ac t i ons
Both technologies undergo the same chemical and electrochemical reactions. The rates of some of these reactions are different (see 4.3).
5 . 2
Cons t r uc t i onma t e r i a l s
Most of the materials used in vented and VRLA cells are the same. Notable exceptions are absorbent separators used in absorbed electrolyte VRLA cells and gelling agents used in gelled electrolyte VRLA cells. Also, flooded cell jars (containers) are typically transparent whereas jars for VRLA cells are usually opaque.
5 . 3 Agi ngandf ai l ur emechani sms ( Eddi eDa vi st opr o vi deaddi t i onal s ubs ec t i ons onaddi t i ons al f ai l ur eandagi ngmodes[ onl yt hos ewhi c har ev a l i donne wermodel sof VRLA] .Someo ft hes emec hani s mst ha twi l l bee x pandedi nc l ude:s eal / c ont ai ner / v a l v e f ai l ur e,negat i v edi s hc ar ge/ depol ar i z at i on,s of ts hor t s ,l os sofc ompr es si on/ bul gi ng, s t r at i fi cat i on,r i ppl ec ur r ent ,et c .Eddi ewi l l al s odos omer ewr i t i ngoft hi ssec t i on) The familiar failure mechanisms of positive plate corrosion and growth, active material failure, post seal leakage, and jar/cover seal leakage associated with vented cells are also present in VRLA cells. However, in VRLA cells, the effects of leakag e at the post and/or jar cover seals are more significant than in vented cells. First, such leakage can contribute to dryout (see 5.3.1) and second, such leaks can allow atmosphericoxygen to enter the cell, which causes discharge of the negative plates. CAUTION
Electrolyte leakage may lead to ground paths and potential fire/explosion hazards.
VRLA cells are susceptible to two failure modes that are normally not associated with vented cells. These are dryout and thermal runaway.
0 Dr yout VRLA cells operate on a precise balance of electrolyte and active material to optimize capacity and oxygen recombination. If the cells are relatively free of oxidizable impurities and the positive grid corrosion rate is not unreasonably high (see 4.6), very little water will be lost through electrolysis. If water is lost, internal resistance increases and performance on discharge degrades. Positive grid corrosion and water loss increase with increasing temperatures. Operation at voltages above the recommended float voltage will also accelerate water loss via electrolysis. Another dryout mechanism that can be significant at elevated temperatures is water loss via diffusion through the jar and cover walls. Dryout by diffusion is highly dependent upon the selection of jar and cover materials., the thickness thereof, operating temperature, and relative humidity of the external environment.
1 Ther malr unaway When a VRLA cell is operating on float or overcharge in a fully recombinant mode, almost all of the overcharge energy results in heat generation. If the design of the system ands its environment are such that the heat produced can be dissipated and thermal equilibrium can be reached, then there is no thermal runaway problem. However, if the recombination reaction gives rise to a rate of heat evolution that exceeds the rate of heat dissipation, the battery temperature will rise and more current will be required to maintain the float voltage. The additional current results in still more recombination and heat generation that further raises battery temperature, and so on. The net effect can be accelerated dryout and/or melting of the battery. This potential problem is further aggravated by elevated ambient temperatures or by cell or charging system malfunctions. The possibility of thermal runaway can be minimized by use of appropriate ventilation between and around the cells and by limiting the charger output current and voltage such as by using temperature-compensated chargers. In the gelled electrolyte system, the gel has intimate contact with the plates and the container walls and provides better heat dissipation characteristics than the absorbed electrolyte system, but not as good as the vented (flooded) system [B4].
7. 2
El ect r ol yt econcent r at i on
By design, absorbed electrolyte VRLA cells have lesser volumes of electrolyte than vented cells of equivalent dimensions. This would normally result in lower long-duration capacities for the absorbed electrolyte cells of equivalent dimensions. Gelled electrolyte cells will usually have the same volume of electrolyte as vented cells. However, the presence of the gel interferes with electrolyte diffusion and convection, which results in reduced highrate capacities. In both cases, the expected reductions in capacity may be partially compensated for by increasing the electrolyte concentration (specific gravities). (Note that higher specific gravities increase the rate of corrosion; i.e., speeds up aging.)
7 . 3
Char gi ng vol t a ge
The electrolyte specific gravity that is typical of VRLA cells may require higher float voltages. The actual float voltage depends upon the specific gravity of the electrolyte being used. Recommended float voltages are provided by the battery manufacturers. Garth Corey will rewrite this section to include other charging regimes besides float.
7 . 4
Abs enc eoff r e ee l e ct r ol y t e
Designs of VRLA cells are such that the electrolyte should be fully immobilized and thus little, if any, liquid electrolyte should leak from the cell in the event of a jar and/or cover break or crack. The absence of free electrolyte simplifies the handling of VRLA cells. It also means that spill containment is unnecessary (see IEEE 1187 for further information on this topic).
7. 5
Mai nt enance andt e st i ng
By the very nature of the designs of VRLA cells, specific gravity readings and water additions are not practical. Consequently, the batteries are often referred to as “maintenance free.” However, this is true only with respect to the electrolyte. Otherwise, VRLA battery maintenance is similar to that required for vented batteries, and will require special procedures to compensate for the inability to determine the amount and concentration of electrolyte. (See IEEE Std 1188-1996.) Due to their design and historical reliability (see Section 5.10) VRLA batteries may require increased monitoring and testing (see IEEE standards 1188 and 1491).
7 . 6
Or i e nt a t i oni nus e
Unlike vented lead-acid cells where the only possible orientation requires that the cell cover be facing upward, certain VRLA cell designs allow for operation in other orientations. Contact the manufacturer for specific instructions.
7. 7
Vent edgas
In a VRLA cell operating in a fully recombinant mode, there will be a slow buildup of gases. Eventually, the composition of this gas will be essentially pure hydrogen. When the cell internal pressure exceeds the valve release pressure, the gases will be vented into the atmosphere. Adequate ventilation must be provided in order to prevent buildup of hydrogen gas in the area. CAUTION When a VRLA cell is charged at voltages above its recombinant limits, potentially explosive mixtures of hydrogen and oxygen are generated within the cell and released at rates similar to those of vented cells. Contact the manufacturer for information on gassing rates and recombination efficiencies.
7. 8
Hi st or i calRel i abi l i t y
This section to be provided by Eddie Davis
7. 9
VRLA advant agesanddi sadvant ages
Much has been written about the drawbacks of VRLA batteries. However they do have advantages over other battery types (most specifically, vented lead-acid cells). Some of the advantages are listed below: •
Low Initial Cost Compared to many other technologies (this refers not only to other battery technologies, but also includes other types of backup, such as flywheels, fuel cells, etc.), VRLA batteries typically cost the least for equivalent energy supply levels. However, the total life cycle cost of ownership (including maintenance, replacement, operating costs, etc.) must be considered. Many applications (especially those in adverse operating conditions, like high temperature environments) should have a life-cost study performed to determine the appropriate battery type based on life cycle economics.
•
Compact Compared to flooded lead acid batteries, VRLAs are relatively compact. They can be placed in smaller "confined" (but ventilated) spaces. Because of the way they are manufactured and packaged, VRLAs can be made in sizes much smaller than those typically found for flooded VLA cells.
•
No Watering Because they do not normally require watering, VRLAs can "survive" with less maintenance than flooded lead-acid cells. However, this does not excuse the user from maintenance as defined in IEEE 1188. Those who don't do maintenance open themselves up to a significantly increased risk of failure.
•
Easier Installation VRLA batteries are typically less costly to install than flooded batteries, and can typically be installed in less time than equivalently sized flooded batteries.
•
"Non-Spillable" (Transport and Placement) VRLA batteries that have passed the appropriate qualification tests can be classified by the U.S. Dept. of Transportation (DOT) as UN2800 - Batteries, NonSpillable. Although they can leak very small amounts of electrolyte, these VRLAs qualified as UN2800 are allowed to be transported by plane. They are also allowed to be transported for relatively short distances by just about anyone (a special transport contractor doesn't have to be hired to visit 100 cabinets [with 4-8 batteries each] in an 18-wheeler and then take the used batteries to a recycling faciltity; these small batteries can be centralize them first). Because they are essentially "non-spillable", most of them can be placed in more convenient space-saving configurations (such as front terminal, or "horizontal"). By most interpretations, VRLAs are exempt from costly spill containment mandated by Fire Codes.
As defined elsewhere in this document (and others), the major disadvantages of VRLAs are shown below. Note that the poor environments VRLAs are placed in contribute greatly to many of these "disadvantages". •
Thermal Issues VRLA batteries are susceptible to potentially destructive and dangerous thermal runaway. Similar to flooded lead acid batteries (but perhaps exacerbated by the thermal runaway phenomenon), VRLAs have short lifetimes in high temperature environments. There are other technologies (e.g., Ni-Cad) which perform much better in high temperature evironments.
•
Short Life VRLA batteries (even when kept in similar environments) have much shorter average lifetimes than flooded lead-acid batteries, Ni-Cads, and other competing technologies. (Failure modes are discussed elsewhere in this document.)
•
Lower Reliability Due to the factors just mentioned, use of VRLA batteries typically lowers the reliability of an application (in comparison to flooded lead acid batteries, for example).
6 .Se l e ct i ngVRLAba t t e r i e s The following factors should be considered in selecting VRLA batteries.
7. 1
Temper at ur e
Elevated temperature operation will shorten VRLA battery life. As a general rule, prolonged use at elevated temperatures will reduce the battery life by approximately 50% for every 8 °C (15 °F) above 25 °C (77 °F). For VRLA batteries, optimal gas recombination is a function of operating temperature. Therefore, battery charging voltage must take temperature into consideration in order to maximize battery life. The additional issues of possible dryout and thermal runaway with VRLA batteries should be carefully assessed for elevated temperature operation. Consideration should also be given to “fail-safe” current limits on chargers for elevated temperature applications. In addition, some VRLA battery manufacturers recommend the use of temperature-compensated chargers for those applications where significant temperature variations can be expected. Monitoring of temperature (both ambient and battery) can help warn of possible problems (see IEEE Std. 1491).
7. 2
Char gi ngl i mi t at i ons
In general, VRLA batteries, because of their higher electrolyte specific gravities, will require a higher float voltage per cell. Therefore, charger capability and system voltage limitations must be considered. Some systems that normally would have been engineered with 24 vented cells may be engineered with 23 larger VRLA cells, thus maintaining capacity while solving the float voltage issue. As mentioned in section 5.3.2, temperature compensation of charging, or other methods of limiting recharge current, should be strongly considered for all VRLA applications. Garth Grey and Sam Norman have volunteered to add to and modify this section to allow for other charging regimes besides float.
7 . 3
Spa cel i mi t a t i ons
The ability to orient some VRLA cells in nonconventional orientations permits maximizing battery capacity in certain space-restrictive applications. Also, because of increased electrolyte specific gravity and other factors, VRLA batteries may have greater volumetric energy density than their flooded counterparts, which could provide space savings. Note that VRLA manufacturers can make their batteries even smaller and less expensive by making thinner plates. However, this costs a price in battery lifetime. Another factor that somewhat limits the space savings of VRLAs is the need for spacing of VRLAs for ventilation, maintenance, and replacement (see sections 6.7 and 6. 8).
7. 4
Envi r onment
Properly designed and operated VRLA batteries emit less gas than vented batteries under normal conditions. This feature should be considered in conjunction with the environment in which their use is intended. As always, the effects of cell or equipment malfunction must also be considered. (See IEEE Std 1187-1996 and IEEE Std 11881996.) Humidity is another consideration for VRLAs. As mentioned in section 5.3.1, dryout is an issue with these batteries. Humidity can play a minor role in this process. Some plastics are more porous than others to diffusion of hydrogen (the smallest molecule). With the more porous plastics, low humidities can speed up this very small rate of diffusion. However, this is generally a minor issue, and high humidities are often not good for electronic equipment that may be in the same area. Other environmental considerations include active or passive ventialtion for gas management (see IEEE Std. 1187 for hydrogen evolution rates, plus section 6.6 below), thermal management (see section 6.7), and temperature (see section 6.1).
7. 5
Li f econsi der at i ons
Battery life versus system design life should always be considered in the selection process. In general, VRLA batteries are more sensitive to abusive conditions of temperature, voltage, and current than vented (flooded) batteries. Also, historical reliability, even in good environments shows a shorter average lifetime for VRLA cells (see Section 5.10).
7. 6
Saf e t y
As with all lead-acid batteries, adequate ventilation must be provided for VRLA batteries (never install a VRLA in a sealed environment). In addition, it is strongly recommended that VRLA cells be equipped with flame arrestor devices. Electrically, VRLA batteries pose the same hazards as vented batteries. VRLA plastics can be flame retardant or not (typical flame retardancy standards include UL 94-V0 and LOI 28). There are drawbacks to flame retardant plastics (they are typically softer, etc.). Many of these drawbacks can be overcome by case design or containment of the case. For more information on safety considerations for installing and maintaining VRLA batteries, see Section 4 of IEEE Stds. 1187 and 1188, respectively.
7. 7
Ther malmanagementconsi der at i ons
As noted in section 6.1, VRLA batteries are particularly sensitive to high temperatures and overheating (which can lead to thermal runaway). A few simple measures or selection considerations can reduce the risk of overheating and thermal runaway. •
Charging techniques and voltages can affect battery temperature (see section 6.2)
•
Forced air cooling is the best thermal management vehicle. In installations without cooling, forced air ventilation will help. As a minimum, cells or monoblocks should be spaced at least 10 mm apart to allow for natural convection (they should also be spaced apart to allow for forced air cooling or ventilation airflow). Choose battery stands or trays that allow and/or force this spacing. Because the battery also loses heat through the posts, air space on top of the cells is also helpful.
•
Gel cells have better thermal transfer to the outside walls of the battery than AGM cells (although AGM cells have other advantages, as noted in section 6.9).
•
For monoblocks, designs that expose more surface area of more cells to the outer walls of the battery have better heat transfer. For example, a 2x3 cell configuration in a 12 V monoblock is typically better than a 1x6 configuration.
•
Homogeneity of temperatures across a string (typically, no more than a 3 C difference should be seen from the hottest to coolest cell/monoblock in a string) are a consideration when designing ventilation. °
7 . 8
Ma i nt e na nc ec ons i de r a t i ons
Maintainability and replaceability of the battery must be considered when selecting the battery and it's stands/cabinets/trays/racks. (See also IEEE Std. 1188.) Most VRLA maintenance is done on or near the terminal posts. Front terminal batteries are the easiest to maintain in relay racks or cabinets. For top terminal batteries in these applications, slide-out trays (rated for the seismic zone in which they are installed) can help. (With slide-out trays there needs to be top space to ensure that there is no inadvertent shorting of posts or intercell cables when the tray is pulled out.) If slide-out trays are not an option, at least 100 mm (or more if possible) of space should be left on top to access the terminal posts. The actual top spacing will agree with the number in IEEE Std. 1187
VRLAs are often chosen because they will fit in spaces that flooded batteries will not. Spacing for maintenance and thermal management will help the issue, but be sure not to buy cells/monoblocks that are very unique in size (size as in height, length, and width). If that battery is ever discontinued or found to have major problems, the user may be in trouble if it is the only battery size that will physically fit.
7. 9
Choosi ngAGM orgelcel l s
AGM and gel cell technologies are both used. Which one is picked has historically depended on what the industry typically used historically. There should be more concrete criteria for deciding which technology is better for the application (they both have advantages and disadvantages). Since AGMs are the most common and most familiar to most users, the followingl comments will focus on what makes gel cells different (and better or worse) than AGMs from a performance perspective. •
Gel cells will not suffer from decompression. AGM cells depend on contact of the matte with the plates. If the plastic case of a VRLA battery "relaxes" or "bulges" too much due to gassing, heat, etc., the AGM cell runs the risk of losing compression (this contact between the matte and the plate), which leads to reduced performance and/or failure.
•
As mentioned in the previous section on thermal management, gel cells tend to conduct heat away from the plates to the outside of the batteries better than do AGM cells, partly because they have more electrolyte. This reduces (but definitely doesn't eliminate) the risk of thermal runaway.
•
In the first year or more of life, gel cells will tend to recombine less efficiently than AGMs, until they have developed the internal fissures/cracks in the gel necessary for recombinant gas transport. (In fact, gel cells will never recombine as efficiently as a properly designed AGM cell, all the way through the life.) This means that they will gas more. That doesn't necessarily mean they will dry out sooner, since they start with more electrolyte than the AGM cells. The building of the internal fissures may cause inconsistent ohmic readings in the first year or so of life of a gel cell. This means that baseline ohmic readings recommended by IEEE Std 1188 may not be as valuable for gel cells as they are for AGM cells.
•
Gel cells have higher internal resistance than AGMs, simply due to the fact that their electrolyte is in a more immobilized form. This means that for super high-rate discharge applications, they may not be the best choice.
•
Not all gel cells are designed to be laid in horizontal or front-terminal positions. Consult the manufacturer.
•
The gelled electrolyte system may provide better deep-discharge rechargeability.
7 . 1 0
Appl i c at i onsa ndpl a t ede si gn
Plate grid alloys (e.g., lead-calcium, pure lead, lead-antimony, etc.) affect how that battery will perform under certain types of conditions. Other plate design parameters (such as thickness), also affect how the battery will perform (for example, thinner plates tend to give better high-rate discharge performance for the same physical size and cost, but have shorter life). The user needs to take these factors into consideration when choosing a battery. Some considerations are listed below. •
If the battery will cycle a lot (in a photovoltaic application for example), the user would want to steer clear of pure lead plate grids. Choose a battery specified by the manufacturer as a cycling type battery (for example, batteries with lead-antimony plate grids tend to cycle well). The manufacturers publish data on typical cycles expected out of batteries. (Note that many high-antimony content plate designs, which are commonly known as lead-antimony plates, may not work well in float applications. Low-antimony plate designs, which are commonly known as lead-selenium plates, may not suffer from this problem, and may work well in both applications. Consult the battery manufacturers for the best plate design if the application requires frequent cycling.)
•
Pure lead grids function better in lower specific gravity designs than in higher specific gravity designs. Although pure lead grids have superior life on float, as mentioned, they generally are not good for frequent cycling or deep-discharge cycling.
•
UPS applications are one example of applications that typically have a high discharge rate. Grid alloys and grid thicknesses are important factors for high discharge rate applications. Unless you want to pay significantly more, you might want to choose a battery that is rated in Watts (or kW) per cell. This typically signifies that the manufacturer has designed that battery for high discharge rate applications.
Bruce Cole has offered to put this section 6.10 in more of a matrix format.
7 .Si z i ngVRLAba t t e r i e s There is nothing unique about sizing VRLA batteries for stationary applications as compared to sizing practices applied for vented batteries. The "Determining Battery Size" clause of IEEE Std. 485 applies equally to VRLA batteries for stationary applications.
7. 1
Par a l l elSt r i ngs
Using parallel strings can greatly increase the reliability of a system by mitigating against the failure of individual cells or strings. For long duration disharges, simply splitting the required ampere-hour reserve over 2 or more strings is usually sufficient. However, for high-rate discharge applications, redundancy of strings may be a better option. In addition, parallel strings improves maintainability. If the strings are installed with disconnects, one of the parallel strings can be easily disconnected for greater safety in maintenance without seriously compromising the backup ability of the whole battery system.
7. 2
Mi xi ngvent edandVRLA
Typically, vented and VRLA batteries have different specific gravities and internal resistances (thus different charging characteristics). This means that VRLAs and vented cells should never be mixed in the same string. Theoretically, VRLA battery strings can be paralleled with vented lead-acid strings. However, this is not usually done, as there is not a lot of experience with it; plus differing numbers of cells (typically fewer) must usually be used in the VRLA string, and it's difficult to match the optimum charging voltages/regimens.
7 . 3
Ov er s i z i ng
IEEE Std. 485 has built-in factors in the sizing calculations to allow for aging. In addition, 5-10% additional oversizing can allow a very quick recharge (since the first 90-95% of charge capacity is usually quickly returned to the battery within a day or less, depending on the charger size; while the final 5-10% may take days to weeks to return to the battery).
