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Lead Acid Battery Basics

Flooded or wet cell, Absorbed Glass Mat (AGM), gel cell, and Sealed Valve Regulated Lead Acid (SVRLA) are all types of lead acid storage batteries. They all work on the same chemical principal, namely the reaction of sulfuric acid with metallic lead and lead oxide to form lead sulfate.

Battery Types
Charging and Discharging
Type Comparison
The Numbers

Battery Types - First a brief description of the various types of lead acid batteries and their main characteristics.

Later more on the specifics of charging and discharging lead acid batteries.

Flooded batteries are the oldest type of lead acid battery, and are the most common type of battery used for heavy duty storage applications. These batteries are also known as wet cell batteries, because they are filled with liquid electrolyte. These batteries can be used either for high amperage starting applications or for any type of storage application. Flooded batteries are the least expensive type of storage battery, and in many ways the most forgiving. Flooded batteries are also sometimes referred to as "vented batteries" or "open cell batteries", to distinguish them from sealed batteries which may also use liquid electrolyte.

The term "Sealed Valve Regulated Lead Acid Battery" refers to any lead acid battery that is sealed with pressure valves to minimize the loss of electrolyte during charging, and to prevent spilling of electrolyte. These batteries are also known as recombinant batteries, because the oxygen gas formed during charging recombines with hydrogen ions to re-form water. Most modern automotive starting batteries are of the SVRLA variety, and use thin lead plates in liquid sulfuric acid. These SVRLA starting batteries are almost universally referred to as "maintenance free" batteries because distilled water need not, and indeed cannot be added. Thin plate starting batteries are sometimes also referred to as lead-calcium batteries. Calcium is often used as an alloying agent to improve the structural properties of the lead structure that supports the plates. SVRLA starting batteries are generally high performance batteries that will work well in any application, but extremely thin plates sometimes cause dramatic failure if the battery is routinely discharged deeply.

AGM batteries are also a type of SVRLA battery, but they use a matrix of glass fibers between the lead plates to immobilize the liquid electrolyte. These batteries are also known as immobilized electrolyte batteries and have become very popular recently because they can be used in nearly any orientation, and are somewhat spill resistant even in the event of case failure. AGM batteries hold up in most applications just about as well as any type of lead acid battery and are often fairly resistant to overdischarge damage.

Another type of SVRLA battery is the gel cell battery, which is also a type of immobilized electrolyte battery. Gel batteries use a gelled electrolyte around the lead plates, which makes them able to be used in any orientation. Gel batteries are most well known for their resistance to being damaged by undercharging and their propensity for being destroyed by overcharging.

The newest type of lead acid battery is the open cell AGM battery. These batteries do not use pressure valves and have easily removable caps like traditional flooded batteries.

Charging and Discharging - All types of lead acid batteries use the same chemical reactions to store electrical energy, and therefore have remarkably similar characteristics.

Lead acid battery cells have a nominal voltage of about 2.0V, so a 12V battery has 6 cells. For the purposes of this discussion all voltages will refer to a battery with a nominal voltage of 12V. All lead acid batteries are charged to about 14.4-14.6V. 14.4V is normally an acceptable charge voltage for a lead acid battery of any type. Various types of batteries and applications do however benefit from slightly higher or lower charging voltages. Higher or lower temperatures also affect the optimal charge voltage for lead acid batteries. Most sophisticated charging devices can be set for different types of batteries, and incorporate temperature compensation to automatically adjust the charge voltage based on the temperature of the batteries.

All lead acid batteries have a limited acceptance rate, that is they will only accept a certain amount of amperage during charging. When a battery is deeply discharged it will accept a very high charging current, as the battery charges though the amount of current that it will accept drops off, until when a 100Ahr battery is fully charged it will only be accepting about two to five amps. The temperature of a battery also affects it's acceptance rate, a hot battery will accept more current than a cold battery. But be careful here, just because a hot battery will accept a high current does not mean that it should be charged at that high rate. A temperature compensated charging algorithm will reduce the charging voltage on a hot battery, and therefore reduce the acceptance rate to a safe level.

During discharge all lead acid batteries deliver about 12.6V to 12.8V with a light load and near a full state of charge. When nearly fully discharged the discharge voltage drops to somewhere around 10.5 to 11.5V depending on the design of the battery. The rate of discharge also affects the voltage delivered. Higher discharge currents cause voltage sag, which lowers the voltage delivered. A fully charged battery under heavy load might only deliver 11.5V. Colder temperatures of course reduce a battery's discharge voltage substantially as well.

The capacity of lead acid batteries is expressed in amp hours (Ahr) at the 20 hour rate. A 100Ahr battery will deliver 5A for 20 hours. At higher rates of discharge all lead acid batteries deliver less than their rated capacity. At the one hour rate a 100Ahr battery will only deliver about 50 to 65 Ahr. So if a 100Ahr battery is discharged with a 50A current, it will be dead in just over an hour. This is known as the Pukert effect of lead acid batteries. Cold temperatures reduce the amp hour capacity of a battery, and conversely high temperatures increase the amp hour capacity of a battery.

Over time all lead acid batteries degrade for several reasons. All batteries have a finite cycle life, even under ideal conditions batteries can only be cycled so many times before the lead and lead oxide on the plates become so irregularly distributed that they are not readily available for reaction with the electrolyte. As batteries age they may lose capacity for several reasons, or they may stop working all together. High quality lead acid batteries of any type will do about 500-600 cycles to an 80% depth of discharge (DOD) if cycled at reasonable charge and discharge rates (less than about 30 amps for a 100ahr battery). If a battery is not discharged so deeply it will do more cycles, with flooded batteries slightly edging out other types of batteries for maximum cycle life.

Cycling is not the only thing that causes batteries to age though. If batteries are charged for too long, or at too high a voltage the electrolyte becomes depleted and the capacity of the battery drops. If batteries are stored in a partial state of charge, or are not fully charged they will degrade in a way that is commonly referred to as "sulfation". This is a misleading term since the formation lead sulfate is a normal process within all lead acid batteries during discharge. What people mean when they say that a battery is "sulfated" is actually that the lead sulfate has crystallized, and can no longer be converted back to lead and sulfuric acid during normal charging. The longer lead sulfate sits on the plates the more it hardens into large crystals that resist conversion back to lead, lead oxide and sulfuric acid. The lead plates in batteries also undergo a process sometimes referred to as "corrosion". This is actually a hydration process, where the lead reacts with water to form hydroxides of lead. This hydration of batteries is more rapid when they are deeply discharged, one of the reasons it is a good idea to keep batteries above a 50% state of charge as much as possible. Another side reaction that could take place is the formation of calcium sulfate if calcium is present either intentionally in the form of structural lead-calcium alloy supports for the plates, or as a contaminant in the lead plates themselves. Over discharging can quickly kill any lead acid battery that does not have a large excess of lead, and cheap batteries with a deficit in lead are extremely susceptible to over discharge damage. A large excess of lead to provide total protection against accidental over discharge adds not only substantially to the cost of manufacture of the battery, but also makes it heavier. In the other direction though less lead than would be ideal for a certain size case does save on manufacturing cost, but does not save any weight in the finished product. A battery with a deficit in lead has a substantially reduced capacity, yet weighs hardly less than a regular battery in the same case.

All lead acid batteries are considered to use sulfuric acid for the electrolyte, and the standard specific gravity is 1.35, although lower specific gravity electrolyte may sometimes be found in batteries. Most batteries are specifically labeled as using sulfuric acid, and sulfuric acid has been considered the one and only electrolyte in use. This is however not totally true, as batteries have sometimes come with phosphoric acid as the electrolyte. The main difference is that phosphoric acid batteries are quite a bit more resistant to damage from being left in a partial state of charge. Even after several years at a partial state of charge phosphoric acid batteries will recover most of their capacity, where sulfuric acid batteries do not recover well from just a few months in a partial state of charge. During regular cycling any time over 12 hours spent in a partial state of charge leads to lead sulfate crystallization. Fully charging at least every three days combined with monthly equalization charging can reverse most of this lead sulfate crystallization, and batteries can still easily last three to five years. Robust lead acid batteries that attain a full state of charge every day are capable of lasting for a decade or more. The strength of the electrolyte solution has a great deal to do with the performance of lead acid batteries as well. Standard 1.35 specific gravity sulfuric acid allows most batteries to perform fairly well in terms of maximum current delivery and low voltage sag, with the thickness of the plates greatly affecting high current performance as well. A weaker electrolyte solution will yield slightly lower discharge voltages and lower maximum current delivery even when the battery is fully charged. When a battery with a weaker electrolyte solution is discharged the voltage will drop off dramatically unless the design of the battery provides for a large excess of electrolyte. A large excess of electrolyte works well for nearly all applications, except for the fact that the batteries then need to be larger and heavier to deliver the same capacity.

Type Comparison - Now a bit about what various types of lead acid batteries do well, and what some of them do poorly.

Flooded batteries do nearly everything well. They are low cost, have the highest cycle life of any lead acid battery, and weigh slightly less than other batteries. Flooded batteries are the only type of batteries that can be used with older unregulated charging sources. They can tolerate a certain amount of overcharging because water that is lost to out gassing can simply be replaced. In the case of extreme over charging where the sulfuric acid itself is lost in the form of hydrogen sulfide gas (rotten egg smell), new sulfuric acid can be added. This technique is however almost never used these days. Voltage regulated charging, multi stage charging and temperature compensated charging go a long way to getting the electrolyte to last for the life of the plates. When crystallization of lead sulfate occurs in flooded batteries they take much longer to charge, and capacity will suffer. This process can partially be reversed by applying an "equalize charge". This equalization is intentional over charging of batteries up to about 14.8 to 15.3V, and should be done at about 3 to 8 amps for a 100Ahr battery for several hours at a time. This over charging will cause gassing, so the electrolyte level should be checked during equalizing.

AGM batteries are chemically the same as flooded batteries. The higher performance of AGM batteries is due mostly to the fact that the lead plates are thinner. AGM batteries can provide higher peak current, suffer less from voltage sag, suffer less from the Pukert effect and have a higher charge acceptance rate than flooded batteries. Essentially AGM batteries perform like thin plate starting batteries, except that they tend to hold up much better when deeply discharged. The glass mat supports the thin plates and to some extent inhibits lead sulfate crystallization. AGM batteries cannot be aggressively equalize charged, as any electrolyte lost is either difficult or impossible to replace. Therefore any greater longevity associated with less formation of lead sulfate crystals is offset by the fact that little can be done about what crystallization does take place. As with flooded batteries AGM batteries must be fully charged periodically, and should not be stored in a partial state of charge.

Open cell AGM batteries are essentially the same as traditional flooded batteries, except that good over discharge protection can be provided with less of an excess of lead. The AGM separators do however somewhat interfere with the flow of electrolyte, and the high current performance of the battery may suffer if the surface area of the plates is not increased by using a larger number of thinner plates.

Gel cell batteries are in many ways similar to AGM batteries, except that they can be almost instantly damaged from over charging. If a gel battery is charged to a voltage where it begins to gas, the escaping gas can push the gelled electrolyte away from the plates, forming permanent voids, and reducing capacity. This is mostly a problem if gel batteries are used with older poorly regulated charging equipment. With a properly regulated charging source gel batteries can last for many years in demanding applications. Gel batteries are the most tolerant of deep discharges and undercharging of any type of lead acid battery. Phosphoric acid added to the electrolyte of nearly all gel batteries contributes to their resistance to lead sulfate crystallization. Nevertheless most gel batteries still need periodically to be fully charged to maintain their capacity. Gel batteries are the most expensive type of lead acid battery, and also have a slightly lower energy density. Gel batteries also are the poorest performers in high discharge situations, exhibiting large voltage sag, and a more pronounced Pukert effect. Expensive gel batteries are the most likely to have a large excess of lead for good overdischarge protection, but any type of storage battery may be available with a large excess of lead.

The Numbers - And finally some quantitative information on the differences between the types of batteries

Cycle Life - Manufacturers of deep cycle batteries usually provide information about how many cycles their batteries will do under ideal charge discharge conditions. One important thing to keep in mind is that batteries very often fail not because they have reached their theoretical cycle life, but rather because they have not been managed properly. Gel and AGM batteries are not rated to do as many deep cycles as flooded batteries, yet in practice gel and AGM batteries often last longer in the same application. This is usually due to cheap flooded batteries with a deficit in lead being over discharged. Typical data for cycle life at 80% depth of discharge would be 500 cycles for AGM, 600 cycles for Gel and 700 cycles for flooded. The gist of these numbers is that the rated cycle life of the different types of batteries is rather similar. Thicker lead plates increases the cycle life of any of these lead acid batteries, but thick plates mean poorer performance and less capacity for a given weight and size. At a shallow depth of discharge batteries are rated to do thousands of cycles. Cycle life of batteries is approximately proportional to depth of discharge. That is if a battery is discharged only half as deeply, it will do a bit more than twice as many cycles. If a battery is cycled to 100% depth of discharge though, its cycle life is disproportionately lower. Never discharging bellow 80% is considered a good idea, and never cycling below 50% not only reduces degradation, but reduces expectations as well. Batteries can only attain their theoretical cycle life if they are charged and discharged at reasonable currents. This is probably discharging at about the five or ten hour rate and charging at about 0.1 to 0.2C. Near maximum cycle life can however be attained at somewhat higher discharge currents. Discharging at the two hour rate a flooded battery can still deliver over half of its rated number of cycles. Higher currents or time spent in a partial state of charge will shorten the life of the battery dramatically.

Energy Density - Energy density is the energy deliverable by a battery divided by its weight. Energy density is typically expressed in watt hours per kilogram (Whr/kg). A lower energy density means that the battery will be heavier. The energy density of all lead acid batteries is remarkably similar, with flooded batteries being the lightweights by a small margin. Gel batteries have an energy density of about 37 Whr/kg, AGM comes in at about 40 Whr/kg, and flooded batteries typically deliver 46 Whr/kg. These numbers are for the twenty hour rate, higher discharge rates yield significantly lower energy densities. Batteries with a deficit in lead deliver significantly lower energy densities than these. Energy density on a mass basis is usually the most significant characteristic of a battery, but energy density per unit volume is also sometimes of interest. The type, shape and application of lead acid batteries has a great deal to do with energy density on a volume basis. Short flooded batteries that need to have good resistance to spilling when tilted over at an extreme angle are large for a particular capacity because the plates must be rather far down from the top of the battery. If this type of battery does not have a deficit in lead then it can simply be filled to the top with strong electrolyte to increase it's performance in a stationary application.

Pukert Effect - Lead acid batteries typically deliver about 50-70% of their 20 hour rate capacity when discharged at the 1 hour rate. The difference in performance at high discharge rates is mostly dependent on plate thickness, thin plate batteries perform best in high discharge situations. Weaker electrolyte also reduces high discharge performance. Since AGM batteries typically have thin plates and strong electrolyte they are the best performers at high discharge rates. Thin plate starting batteries of the flooded type also perform well at high discharge rates. Batteries with thick plates designed for deep discharging and high cycle life have the poorest high discharge rate performance. A reasonably robust flooded storage battery will deliver 55-70% capacity at the one hour rate, while an AGM battery might deliver 62-67% and gels often do about 52-60%. This is not a huge difference, all lead acid batteries perform poorly at high discharge rates. The observed difference in performance between the 20 hour rate and the one hour rate is only partially due to the Pukert effect. Batteries with a large excess of lead and thick plates can support deep discharges at low currents, but are no better than a regular battery at high currents. On the flip side batteries with a deficit of lead appear to perform as well as a regular battery at high currents, but will simply fail when deeply discharged. If the difficulties with excesses and deficits of lead are ignored the true Pukert effect would probably cause a lead acid battery to have a reduced capacity of about 70% of the 20 hour rate when discharged at the one hour rate.

Acceptance Rate - Batteries with thick plates have lower acceptance rates, batteries with thin plates have higher acceptance rates. Gel cell batteries tend to have the lowest acceptance rates, and pressurized liquid electrolyte SVRLA batteries with thin plates have the highest acceptance rates. Because AGM batteries typically have very thin plates they tend also to have high acceptance rates. Even though an AGM battery might accept 3C during charging, it will not accept this high current for very long, and AGM batteries may overheat being charged at this high current. It is usual to limit the charge current for deep cycled batteries of any type to no more than 0.35C, that is 35A for a 100Ahr battery. For maximum cycle life heavy duty storage batteries are normally charged at an even lower rate, about 0.15-0.25C. Nearly any battery will accept 1C charging when it is deeply discharged, but charging at 1C does not shorten the overall charging time by very much, this is because the acceptance rate of batteries drops off as they are charged. By the time a battery is up to a 75% state of charge it will only take about 0.25-0.35C. Charging the last 25% takes an hour or more regardless of how much amperage the charging source is capable of delivering, this is called taper charging. Thinner plates provide a higher acceptance rate and also reduce the taper charging time. For equal plate surface area flooded batteries would have the highest acceptance rate and gel batteries would have the lowest acceptance rate.

Charge Voltages - At normal room temperature, 70 degrees Fahrenheit, batteries are charged to about 14.6V. Gel batteries require a lower voltage, about 14.1-14.4V. AGM batteries can be charged at anywhere between 14.3 and 14.8V depending on application. At higher temperatures lower voltages are required, 14.0V in a hot engine room, and in freezing climates 15.0V can be required. If temperature compensation is not used, then charge voltages are normally set based on the typical ambient temperature, and erring on the low side is a good idea. If the charge regulating device does compensate charge voltage based on battery temperature, then slightly higher charge voltages for fast charging can be used. This is especially true for AGM batteries, whose tapper charging times can be reduced at higher voltages provided good temperature compensation is used to protect the batteries from overheating. All lead acid batteries heat up when being charged. High charge rates and high charge voltages can lead to thermal runaway, where the battery continues to heat up until the electrolyte begins to boil. Boiling electrolyte is an extreme event, and should not be confused with the normal hydrolization of water into hydrogen and oxygen gas which occurs during charging. Temperature compensated charging will prevent thermal run away by reducing the charging voltage as the batteries heat up. If charge voltages are too low, or charging time is too short, then any type of lead acid battery will suffer from lead sulfate crystallization and lose capacity. In general, fully charging a lead acid battery requires one to two hours of taper charging once the voltage has come up to the set point. Thin plate starting batteries and some thin plate AGM batteries require considerably less taper charging than this when they are new and in good condition. Slightly low charging voltages are perfectly acceptable, provided that the charging time is sufficient to bring the battery up to a full state of charge. Low charging voltage can easily add an hour to the taper charging period. A lead acid battery is usually considered fully charged when it will accept 3 to 5% of C, lower charge voltages may require the battery be charged until it will only accept 2 to 3% of C. Lead acid batteries generally require relatively short tapper charge times when they are new, and longer tapper charging as they age. If the charge voltages are too high on gel batteries they will simply stop working. AGM batteries are more tolerant of over voltage than gel cells, but will lose capacity if electrolyte is lost due to overcharging. If a flooded battery is charged at too high a voltage it will use water, but it will also slowly lose capacity due to conversion of sulfuric acid to hydrogen sulfide gas (rotten egg smell).

Watering and Equalization - Adding distilled water to batteries is often called watering, and is required maintenance for all flooded type batteries. This should not however be an onerous task. If batteries are requiring watering more than twice a year, then the charging system needs attention. A 6V 225Ahr flooded golf cart battery being cycled daily in a hot climate could be expected to use a pint of water every six months. Water consumption much higher than this would be considered excessive. The root cause of high water consumption is too much charging at too high a voltage. Equalizing excessively causes high water consumption, as does charging at high temperatures. Sustained high current discharge may also use more water. In tropical climates flooded batteries should be charged at a lower voltage, perhaps 14.3V, but still need periodically to be equalize charged to 15V. If the batteries are in a hot engine room then a multi stage regulator with temperature compensation is a must. Many hours of engine run time with a voltage regulator that does not switch to a lower float voltage will cause batteries to use an excessive amount of water. A system that causes high water consumption in flooded batteries will most likely destroy sealed type batteries. As flooded batteries age they naturally tend to use more water, this is because old batteries require a longer taper charging period than they did when they were new. Often the first sign that flooded batteries are nearing the end of their life is the need for more hours of taper charging to get them completely charged. Some manufacturers of AGM batteries are starting to recommend infrequent equalization charging to reverse lead sulfate crystallization. Equalization is a particularly tricky operation with sealed batteries, because any electrolyte lost can never be replaced. It could be worth it to sacrifice a bit of electrolyte to get back lost capacity from an AGM battery, but this is a decision not to be taken lightly. Equalization can easily do more harm than good to an AGM battery. Gel batteries probably should never be equalized because of the risk of voids developing around the plates. If gel batteries are normally charged only to 14.1 volts, then occasional, and very carefully charging up to 14.4 or 14.6 volts could be beneficial. Open cell AGM batteries can of course have water added to them as with traditional flooded batteries, but problems with stratification of the electrolyte and slow dissociation of the added water into the AGM matt may require a more specialized watering procedure.

Cost and Application - Flooded batteries are the low price leader by far. Gel and AGM batteries typically cost two to four times as much as flooded batteries. The cost of batteries varies greatly by application and by geographic location. Flooded 6V golf cart batteries are reasonably robust in deep cycle applications, and are the absolute lowest cost solution for small to medium sized systems. At about $55 to $70 for a 225Ahr 6V GC2 (2008 prices), golf cart batteries have been an unbeatable value. Recently the cost of golf cart batteries has been on the rise, and it may take over $100 to buy one. Golf cart batteries cost much less than any other deep cycle battery, and when properly managed are capable of nearly as many cycles as any heavy duty deep cycle battery. Small 12V flooded batteries vary greatly in both cost and durability. A group 27 flooded battery marketed as a RV/Marine deep cycle, or dual purpose battery, and rated at 115Ahr maximum capacity might cost as little as $60, but may have a disappointing cycle life. At an 80% depth of discharge it may do anywhere from a few dozen to a few hundred cycles. These batteries are little more than traditional non-sealed starting batteries re-badged for use in storage applications. A heavy duty group 31 flooded battery also rated at 115Ahr might cost twice as much, but will perform more like golf cart batteries in terms of maximum cycle life. Larger, heavier flooded batteries with thicker plates such as the 6V L16 size cost even more per amp hour, but can provide the best cycle life. The thicker the plates, the better it will hold up to equalization charging when lead sulfate crystallization needs to be reversed. The most robust industrial batteries, with thick plates and rugged double containers, have the potential to last longer than anything else, but the huge investment and huge weight of these batteries makes them an unappealing option for anything but the largest of electrical systems. Gel batteries also are available in a wide range of sizes and plate thicknesses. All gel batteries are very expensive, a 12V 200Ahr gel battery typically costs $600 or more. AGM batteries are much more popular than gel batteries, are available from a larger number of manufacturers, and cost a bit less. A 12V 200Ahr AGM battery could be had for as little as $400, but for a top brand AGM expect to pay nearly as much as for gel. With the cost of flooded batteries such as golf cart batteries on the rise AGM batteries may not look so extremely overpriced as they did in the past.

Conclusion - Flooded batteries are best for most applications because of their low cost and long cycle life. AGM batteries perform better in high current applications with deep discharges because the glass matt holds the thin plates in place better. Gel batteries may provide some of the deep discharge benefits of AGM batteries, but this comes at the cost of generally poor high current performance and susceptibility to damage from high charge voltages. When quantified, the differences between the various types of lead acid batteries are small in magnitude for all characteristics save one, and that is cost. As far as durability goes the two main considerations are irreversible lead sulfate crystallization from time spent in a partial state of charge and rapid catastrophic failure from the overdischarge of lead deficient batteries. Ultimately neither of these main considerations have much to do with the type of lead acid battery (flooded, sealed, AGM or gel) but rather have to do with an inherent limitation of sulfuric acid lead acid batteries and a tendency for battery manufacturers to put as little lead in batteries as they can possibly get away with.

Stoichiometry - How much lead reacts and why lead acid batteries are so heavy.

A 100Ahr 12V lead acid battery only actually reacts 5.1 pounds of metallic lead on the negative plates and 5.8 pounds of lead dioxide on the positive plates with 4.8 pounds of H2SO4 from the electrolyte to deliver the full 100Ahr. That is a total of just 15.8 pounds of materials reacted, so why is it that this 100Ahr 12V battery weighs 60 pounds? The answer is twofold. First of all it is obvious that more lead than reacts is going to be required on the plates. There has to be an underlying support structure for the plates for the battery to be able to do even one cycle to its rated 100Ahr depth of discharge. For long cycle life it is necessary for quite a bit more lead to be present on the plates so that holes do not develop and pieces of the plates do not fall off onto the bottom of the battery. This requires at least several times more lead be in the battery than actually reacts with the electrolyte. The other reason that lead acid batteries have to be so much heavier is that a whole lot of water is required to get the sulfuric acid where it needs to be. If the electrolyte strength is 1.35 specific gravity when the battery is fully charged and the electrolyte is allowed to weaken to 1.10 specific gravity when the battery is fully discharged then no less than 19 pounds of electrolyte is required for the 100Ahr 12V battery. If the electrolyte is less than 1.35 specific gravity at a full state of charge then the total weight of the electrolyte needs to be even higher to deliver 100Ahr. Allowing the electrolyte to drop down to a specific gravity of 1.1 also means poor high current performance at the end of the discharge. If the battery is to be able to deliver good high current performance all the way to the end of it's rated capacity then more electrolyte is required. Even if 30 pounds of electrolyte is used in a 100Ahr 12V battery a whole lot of the 60 pound weight is the extra lead. About 20 pounds more lead and lead dioxide than actually reacts with the electrolyte. A more typical 100Ahr 12V battery with good overdischarge protection and the ability to do many hundreds of deep discharges would have four times more lead and lead oxide than actually reacts with the electrolyte, and would weigh closer to 70 pounds.

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