Lead-acid car battery
|Specific energy||33–42 Wh/kg|
|Energy density||60–110 Wh/l|
|Specific power||180 W/kg |
|Energy/consumer-price||7(sld)–18(fld) Wh/US$|
|Cycle durability||500–800 cycles|
|Nominal cell voltage||2.0 V|
|Charge temperature interval||min. −35 °C, max. 45 °C|
The lead–acid battery was invented in 1859 by French physicist Gaston Planté and is the oldest type of rechargeable battery. Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, its ability to supply high surge currents means that the cells have a relatively large power-to-weight ratio. These features, along with their low cost, makes it attractive for use in motor vehicles to provide the high current required by automobile starter motors.
As they are inexpensive compared to newer technologies, lead-acid batteries are widely used even when surge current is not important and other designs could provide higher energy densities. Large-format lead-acid designs are widely used for storage in backup power supplies in cell phone towers, high-availability settings like hospitals, and stand-alone power systems. For these roles, modified versions of the standard cell may be used to improve storage times and reduce maintenance requirements. Gel-cells and absorbed glass-mat batteries are common in these roles, collectively known as VRLA (valve-regulated lead-acid) batteries.
Lead–acid battery sales account for 40–45% of the value from batteries sold worldwide (1999, not including China and Russia), a manufacturing market value of about US$15 billion.
- 1 History
- 2 Electrochemistry
- 3 Voltages for common usages
- 4 Measuring the charge level
- 5 Construction of battery
- 6 Applications
- 7 Cycles
- 8 Valve regulated
- 9 Sulfation and desulfation
- 10 Stratification
- 11 Risk of explosion
- 12 Environment
- 13 Additives
- 14 Corrosion problems
- 15 Maintenance precautions
- 16 Sizing Nomenclature
- 17 See also
- 18 References
- 19 External links
The French scientist Gautherot observed in 1801 that wires that had been used for electrolysis experiments would themselves provide a small amount of "secondary" current after the main battery had been disconnected. In 1859, Gaston Planté's lead-acid battery was the first battery that could be recharged by passing a reverse current through it. Planté's first model consisted of two lead sheets separated by rubber strips and rolled into a spiral. His batteries were first used to power the lights in train carriages while stopped at a station. In 1881, Camille Alphonse Faure invented an improved version that consisted of a lead grid lattice, into which a lead oxide paste was pressed, forming a plate. This design was easier to mass-produce. An early manufacturer (from 1886) of lead–acid batteries was Henri Tudor.
Using a gel electrolyte instead of a liquid allows the battery to be used in different positions without leakage. Gel electrolyte batteries for any position date from 1930s and even in late 1920s portable suitcase radio sets allowed the cell vertical or horizontal (but not inverted) due to valve design (see third Edition of Wireless Constructor's Encyclopaedia by Frederick James Camm). In the 1970s, the valve-regulated lead acid battery (often called "sealed") was developed, including modern absorbed glass mat types, allowing operation in any position.
In the discharged state both the positive and negative plates become lead(II) sulfate (Template:Chem/atomTemplate:Chem/atom), and the electrolyte loses much of its dissolved sulfuric acid and becomes primarily water. The discharge process is driven by the conduction of electrons from the negative plate back into the cell at the positive plate in the external circuit.
Negative plate reaction:
- Pb(s) + Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom(aq) → Template:Chem/atomTemplate:Chem/atom(s) + Template:Chem/atomTemplate:Chem/atom(aq) + 2e−
Positive plate reaction:
- Template:Chem/atomTemplate:Chem/atom(s) + Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom(aq) + 3Template:Chem/atomTemplate:Chem/atom(aq) + 2e− → Template:Chem/atomTemplate:Chem/atom(s) + 2Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom(l)
The total reaction can be written as
- Pb(s) + Template:Chem/atomTemplate:Chem/atom(s) + 2Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atomTemplate:Chem/atom(aq) → 2Template:Chem/atomTemplate:Chem/atom(s) + 2Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom(l)
The sum of the molecular masses of the reactants is 642.6 g/mol, so theoretically a cell can produce two faradays of charge (192,971 coulombs) from 642.6 g of reactants, or 83.4 ampere-hours per kilogram (or 13.9 ampere-hours per kilogram for a 12-volt battery). For a 2 volts cell, this comes to 167 watt-hours per kilogram of reactants, but a lead-acid cell in practice gives only 30–40 watt-hours per kilogram of battery, due to the mass of the water and other constituent parts.
Overcharging with high charging voltages generates oxygen and hydrogen gas by electrolysis of water, which is lost to the cell. Periodic maintenance of lead-acid batteries requires inspection of the electrolyte level and replacement of any water that has been lost.
Due to the freezing-point depression of the electrolyte, as the battery discharges and the concentration of sulfuric acid decreases, the electrolyte is more likely to freeze during winter weather when discharged.
During discharge, Template:Chem/atomTemplate:Chem/atom produced at the negative plates moves into the electrolyte solution and then is consumed into the positive plates, while Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom is consumed at both plates. The reverse occurs during charge. This motion can be by electrically driven proton flow or Grotthuss mechanism, or by diffusion through the medium, or by flow of a liquid electrolyte medium. Since the density is greater when the sulfuric acid concentration is higher, the liquid will tend to circulate by convection. Therefore a liquid-medium cell tends to rapidly discharge and rapidly charge more efficiently than an otherwise similar gel cell.
Voltages for common usages
|This section does not cite any references or sources. (February 2012)|
These are general voltage ranges per cell:
- Open-circuit (quiescent) at full charge: 2.10 V
- Open-circuit at full discharge: 1.95 V
- Loaded at full discharge: 1.75 V
- Continuous-preservation (float) charging: 2.23 V for gelled electrolyte; 2.25 V for AGM (absorbed glass mat) and 2.32 V for flooded cells
- All voltages are at Script error: No such module "convert"., and must be adjusted for temperature changes. The open-circuit voltage cannot be adjusted with a simple temperature coefficient because it is non-linear (coefficient varies with temperature). See voltage vs. temperature table.
- Float voltage recommendations vary among manufacturers.
- Precise float voltage (±0.05 V) is critical to longevity; insufficient voltage (causes sulfation) is almost as detrimental as excessive voltage (causing corrosion and electrolyte loss)
Typical (daily) charging: 2.37–2.4 V (depending on temperature and manufacturer's recommendation)
- Equalization charging (for flooded lead acids): 2.5–2.67 V (5A per 100Ah, Battery temperature must be absolutely monitored very closely, check manufacturers recommendation)
- Charging in sulfated state (stored discharged for days or weeks) not accepting small charge current: > 3 V (only until a charge current is flowing)
- Charging in sulfated state: up to 2.6–2.66 V 
- Discharging in sulfated state: 1.6 V (when charging at low rates doesn't improve, discharge rate approximately 5A per 10Ah)
- Gassing threshold: 2.415–2.48 V for sealed, 2.41 V for PzS, 2.36–2.41 V for GiS, PzV, GiV (the value is manufacturer specific, gas is always produced even in storage, 99% of the gas production recombines under normal charging conditions, the higher the voltage exponentially more gas is produced: from 2.3 to 2.5 is factor 1 to > 20, charging above the gassing voltage with high charging current the side reaction will occur enhanced)
Measuring the charge level
Because the electrolyte takes part in the charge-discharge reaction, this battery has one major advantage over other chemistries. It is relatively simple to determine the state of charge by merely measuring the specific gravity (S.G.) of the electrolyte; the S.G. falls as the battery discharges. Some battery designs include a simple hydrometer using colored floating balls of differing density. When used in diesel-electric submarines, the S.G. was regularly measured and written on a blackboard in the control room to indicate how much longer the boat could remain submerged.
The battery's open-circuit voltage can also be used to gauge the state of charge. If the connections to the individual cells are accessible, then the state of charge of each cell can be determined which can provide a guide as to the state of health of the battery as a whole.
Construction of battery
The lead–acid cell can be demonstrated using sheet lead plates for the two electrodes. However, such a construction produces only around one ampere for roughly postcard-sized plates, and for only a few minutes.
Gaston Planté found a way to provide a much larger effective surface area. In Planté's design, the positive and negative plates were formed of two spirals of lead foil, separated with a sheet of cloth and coiled up. The cells initially had low capacity, so a slow process of "forming" was required to corrode the lead foils, creating lead dioxide on the plates and roughening them to increase surface area. Initially this process used electricity from primary batteries; when generators became available after 1870, the cost of production of batteries greatly declined. Planté plates are still used in some stationary applications, where the plates are mechanically grooved to increase their surface area.
Camille Alphonse Faure invented a pasted-plate construction now typical of automotive batteries. Each Faure plate consists of a rectangular lead grid alloyed with antimony or calcium to improve the mechanical characteristics. The holes of the grid are filled with a paste of red lead and 33% dilute sulfuric acid. (Different manufacturers vary the mixture). The paste is pressed into the holes in the grid, which are slightly tapered on both sides to better retain the paste. This porous paste allows the acid to react with the lead inside the plate, increasing the surface area many fold. Once dry, the plates are stacked with suitable separators and inserted in the battery container. An odd number of plates is usually used, with one more negative plate than positive. Each alternate plate is connected.
The positive plates are the chocolate-brown color of lead dioxide, and the negative are the slate-gray of "spongy" lead at the time of manufacture. In this charged state the plates are called "formed".
One of the problems with the plates is that the plates increase in size as the active material absorbs sulfate from the acid during discharge, and decrease as they give up the sulfate during charging. This causes the plates to gradually shed the paste. It is important that there is room underneath the plates to catch this shed material. If it reaches the plates, the cell short-circuits.
The paste contains carbon black, blanc fixe (barium sulfate) and lignosulfonate. The blanc fixe acts as a seed crystal for the lead–to–lead sulfate reaction. The blanc fixe must be fully dispersed in the paste in order for it to be effective. The lignosulfonate prevents the negative plate from forming a solid mass during the discharge cycle, instead enabling the formation of long needle–like crystals. The long crystals have more surface area and are easily converted back to the original state on charging. Carbon black counteracts the effect of inhibiting formation caused by the lignosulfonates. Sulfonated naphthalene condensate dispersant is a more effective expander than lignosulfonate and speeds up formation. This dispersant improves dispersion of barium sulfate in the paste, reduces hydroset time, produces a more breakage-resistant plate, reduces fine lead particles and thereby improves handling and pasting characteristics. It extends battery life by increasing end-of-charge voltage. Sulfonated naphthalene requires about one-third to one-half the amount of lignosulfonate and is stable to higher temperatures.
Practical cells are not made with pure lead but have small amounts of antimony, tin, calcium or selenium alloyed in the plate material to add strength and simplify manufacture. The alloying element has a great effect on the life and water consumption of the batteries. Antimony-alloyed plates provide longer life but calcium-alloyed plates are much preferred over antimony for their eight times lower water consumption.
Due to the high price of antimony, nearly all car batteries, valve-regulated sealed batteries, and most other non-industrial batteries have been made with lead–calcium grids since the early 1990s and possibly earlier. Tin was added to lead–calcium cells to reduce the corrosion and open-circuit effect. (The calcium oxidizes whenever the positive plate voltage is lower than 40–80 mV above open-circuit and forms an insulator between the active material and the grid. Pulse treatment can help to recover oxide-affected batteries.) Tin helps to reduce this corrosion, but the benefit was deemed not worth 20 USD/lb, and many manufacturers dropped the tin. It is now rare to find antimony cells, with exception of motive power. It is possible to make lead–antimony positive plates and lead–calcium negatives, however, antimony is plated out onto the negatives and this causes the water-saving benefit of calcium negatives to be lost. While it can be argued that calcium saves water and maintenance costs, it is almost never worth cutting the battery life from 20 years to 5.
About 60% of the weight of an automotive-type lead–acid battery rated around 60 A·h (8.7 kg of a 14.5 kg battery) is lead or internal parts made of lead; the balance is electrolyte, separators, and the case.
Separators between the positive and negative plates prevent short-circuit through physical contact, mostly through dendrites ("treeing"), but also through shedding of the active material. Separators obstruct the flow of ions between the plates and increase the internal resistance of the cell. Wood, rubber, glass fiber mat, cellulose, and PVC or polyethylene plastic have been used to make separators. Wood was the original choice, but deteriorated in the acid electrolyte. Rubber separators are stable in battery acid. Rubber provides valuable electrochemical advantages that other materials cannot.
An effective separator must possess a number of mechanical properties; such as permeability, porosity, pore size distribution, specific surface area, mechanical design and strength, electrical resistance, ionic conductivity, and chemical compatibility with the electrolyte. In service, the separator must have good resistance to acid and oxidation. The area of the separator must be a little larger than the area of the plates to prevent material shorting between the plates. The separators must remain stable over the battery's operating temperature range.
Absorbed glass mat
|This section contains instructions, advice, or how-to content. (April 2015)|
In the absorbed glass mat design, or AGM for short, the spacer between the cells is replaced by a glass fibre mat soaked in electrolyte. There is only enough electrolyte in the mat to keep it wet, and if the battery is punctured the electrolyte will not flow out of the mats. Likewise, the mat greatly reduces evaporation, to the point that the batteries do not require periodic refilling of the water. This combination of features allows the battery to be completely sealed, which makes them useful in portable devices and similar roles.
To address the formation of hydrogen gas during discharge, calcium is added to the plates to absorb the gas. This only works during slow discharges, and gas buildup remains a problem when the battery is deeply or rapidly discharged. To deal with these events, AGMs often include a one-way blow-off valve, and are often known as "valve regulated lead-acid", or VRLA, designs.
Another advantage to the AGM design is that the electrolyte becomes the separator material, and mechanically strong. This allows the plate stack to be compressed together in the battery shell, slightly increasing energy density compared to liquid or gel versions. AGM batteries often show a characteristic "bulging" in their shells when built in common rectangular shapes.
The mat also prevents the vertical motion of the electrolyte within the battery. When a normal wet cell is stored in a discharged state, the heavier acid molecules tend to settle to the bottom of the battery, causing the electrolyte to stratify. When the battery is then used, the majority of the current flows only in this area, and the bottom of the plates tend to wear out rapidly. This is one of the reasons a conventional car battery can be ruined by leaving it stored for a long period and then used and recharged. The mat significantly prevents this stratification, eliminating the need to periodically shake the batteries, boil them, or run an "equalization charge" through them to mix the electrolyte. Stratification also causes the upper layers of the battery to become almost completely water, which can freeze in cold weather, AGMs are significantly less susceptible to damage due to low-temperature use.
While AGM cells do not require watering, their recombination process is fundamentally limited by the usual chemical processes. Hydrogen gas will also flow right through the plastic case itself. Some have found that it is profitable to add water to an AGM battery, but you must do it slowly to allow for the water to mix via diffusion throughout the battery. You must also add water before the batteries need it. When a lead acid battery loses water, its acid concentration increases, increasing the corrosion rate significantly. AGM cells already have a high acid content in an attempt to lower the water loss rate, and this brings about short life. If the open circuit voltage of your AGM cells is significantly higher than 2.093 volts, or 12.56v for a 12v battery, you have a higher acid content than a flooded cell; while this is normal for an AGM battery it is not desirable for long life.
AGM cells intentionally overcharged will show a higher open circuit voltage according to the water lost (and acid concentration increased). One Amp-Hour of overcharge will liberate 0.335 Grams of water; some of this liberated hydrogen and oxygen will recombine, but not all of it.
During the 1970s researchers developed the sealed version or "gel battery", which mixes a silica gelling agent into the electrolyte (Silica gel based lead Acid batteries used in Portable Radios from early 1930s were not fully sealed). This converts the formerly liquid interior of the cells into a semi-stiff paste, providing many of the same advantages of the AGM. Such designs are even less susceptible to evaporation and are often used in situations where little or no periodic maintenance is possible. Gel cells also have lower freezing and higher boiling points than the liquid electrolytes used in conventional wet cells and AGMs, which makes them suitable for use in extreme conditions.
The only downside to the gel design is that the gel prevents rapid motion of the ions in the electrolyte, which reduces carrier mobility and thus surge current capability. For this reason, gel cells are most commonly found in energy storage applications like off-grid systems.
"Maintenance free", "sealed" and "VRLA"
Both gel and AGM designs are sealed, do not require watering, can be used in any orientation, and use a valve for gas blowoff. For this reason, both designs can be called maintenance free, sealed and VRLA. However, it is quite common to find resources stating that these terms refer to one or another of these designs, specifically.
Most of the world's lead–acid batteries are automobile starting, lighting and ignition (SLI) batteries, with an estimated 320 million units shipped in 1999. In 1992 about 3 million tons of lead were used in the manufacture of batteries.
Wet cell stand-by (stationary) batteries designed for deep discharge are commonly used in large backup power supplies for telephone and computer centers, grid energy storage, and off-grid household electric power systems. Lead–acid batteries are used in emergency lighting and to power sump pumps in case of power failure.
Traction (propulsion) batteries are used in golf carts and other battery electric vehicles. Large lead–acid batteries are also used to power the electric motors in diesel-electric (conventional) submarines when submerged, and are used as emergency power on nuclear submarines as well. Valve-regulated lead acid batteries cannot spill their electrolyte. They are used in back-up power supplies for alarm and smaller computer systems (particularly in uninterruptible power supplies ("UPS")) and for electric scooters, electric wheelchairs, electrified bicycles, marine applications, battery electric vehicles or micro hybrid vehicles, and motorcycles.
Lead–acid batteries were used to supply the filament (heater) voltage, with 2 V common in early vacuum tube (valve) radio receivers.
Lead-acid batteries designed for starting automotive engines are not designed for deep discharge. They have a large number of thin plates designed for maximum surface area, and therefore maximum current output, but which can easily be damaged by deep discharge. Repeated deep discharges will result in capacity loss and ultimately in premature failure, as the electrodes disintegrate due to mechanical stresses that arise from cycling. Starting batteries kept on continuous float charge will have corrosion in the electrodes which will result in premature failure. Starting batteries should be kept open circuit but charged regularly (at least once every two weeks) to prevent sulfation.
Starting batteries are lighter weight than deep cycle batteries of the same battery dimensions, because the cell plates do not extend all the way to the bottom of the battery case. This allows loose disintegrated lead to fall off the plates and collect under the cells, to prolong the service life of the battery. If this loose debris rises high enough it can touch the plates and lead to failure of a cell, resulting in loss of battery voltage and capacity.
Deep cycle batteries
Specially designed deep-cycle cells are much less susceptible to degradation due to cycling, and are required for applications where the batteries are regularly discharged, such as photovoltaic systems, electric vehicles (forklift, golf cart, electric cars and other) and uninterruptible power supplies. These batteries have thicker plates that can deliver less peak current, but can withstand frequent discharging.
Some batteries are designed as a compromise between starter (high-current) and deep cycle batteries. They are able to be discharged to a greater degree than automotive batteries, but less so than deep cycle batteries. They may be referred to as "Marine/Motorhome" batteries, or "leisure batteries".
Fast and slow charge and discharge
The capacity of a lead–acid battery is not a fixed quantity but varies according to how quickly it is discharged. An empirical relationship between discharge rate and capacity is known as Peukert's law.
When a battery is charged or discharged, only the reacting chemicals, which are at the interface between the electrodes and the electrolyte, are initially affected. With time, the charge stored in the chemicals at the interface, often called "interface charge" or "surface charge", spreads by diffusion of these chemicals throughout the volume of the active material.
Consider a battery that has been completely discharged (such as occurs when leaving the car lights on overnight, a current draw of about 6 amps). If it then is given a fast charge for only a few minutes, the battery plates charge only near the interface between the plates and the electrolyte. In this case the battery voltage might rise to a value near that of the charger voltage; this causes the charging current to decrease significantly. After a few hours this interface charge will spread to the volume of the electrode and electrolyte; this leads to an interface charge so low that it may be insufficient to start the car. As long as the charging voltage stays below the gassing voltage (about 14.4 volts in a normal lead-acid battery), battery damage is unlikely, and in time the battery should return to a nominally charged state.
In a valve regulated lead acid battery (VRLA) the hydrogen and oxygen produced in the cells largely recombine into water. Leakage is minimal, although some electrolyte still escapes if the recombination cannot keep up with gas evolution. Since VRLA batteries do not require (and make impossible) regular checking of the electrolyte level, they have been called maintenance free batteries. However, this is somewhat of a misnomer. VRLA cells do require maintenance. As electrolyte is lost, VRLA cells "dry-out" and lose capacity. This can be detected by taking regular internal resistance, conductance or impedance measurements. Regular testing reveals whether more involved testing and maintenance is required. Recent maintenance procedures have been developed allowing "rehydration", often restoring significant amounts of lost capacity.
VRLA types became popular on motorcycles around 1983, because the acid electrolyte is absorbed into the separator, so it cannot spill. The separator also helps them better withstand vibration. They are also popular in stationary applications such as telecommunications sites, due to their small footprint and installation flexibility.
The electrical characteristics of VRLA batteries differ somewhat from wet-cell lead–acid batteries, requiring caution in charging and discharging.[clarification needed]
Sulfation and desulfation
|This section needs additional citations for verification. (December 2013)|
Lead–acid batteries lose the ability to accept a charge when discharged for too long due to sulfation, the crystallization of lead sulfate. They generate electricity through a double sulfate chemical reaction. Lead and lead dioxide, the active materials on the battery's plates, react with sulfuric acid in the electrolyte to form lead sulfate. The lead sulfate first forms in a finely divided, amorphous state, and easily reverts to lead, lead dioxide and sulfuric acid when the battery recharges. As batteries cycle through numerous discharges and charges, some lead sulfate is not recombined into electrolyte and slowly converts to a stable crystalline form that no longer dissolves on recharging. Thus, not all the lead is returned to the battery plates, and the amount of usable active material necessary for electricity generation declines over time.
Sulfation occurs in lead–acid batteries when they are subjected to insufficient charging during normal operation. It impedes recharging; sulfate deposits ultimately expand, cracking the plates and destroying the battery. Eventually so much of the battery plate area is unable to supply current that the battery capacity is greatly reduced. In addition, the sulfate portion (of the lead sulfate) is not returned to the electrolyte as sulfuric acid. It is believed that large crystals physically block the electrolyte from entering the pores of the plates. Sulfation can be avoided if the battery is fully recharged immediately after a discharge cycle. A white coating on the plates may be visible (in batteries with clear cases, or after dismantling the battery). Batteries that are sulfated show a high internal resistance and can deliver only a small fraction of normal discharge current. Sulfation also affects the charging cycle, resulting in longer charging times, less efficient and incomplete charging, and higher battery temperatures.
Desulfation is the process of reversing the sulfation of a lead-acid battery. It is believed that desulfation can be achieved by high current pulses produced between the terminals of the battery. It is believed[by whom?] that this technique, also called pulse conditioning, breaks down the sulfate crystals that are formed on the battery plates. Pulses must last longer than the resonant frequency of the battery. Short pulses simply feed energy wastefully into the resistive components of this resonant circuit and virtually none into the battery. Electronic circuits are used to regulate the pulses of different widths and frequency of high current pulses. These can also be used to automate the process since it takes a long period of time to desulfate a battery fully. Battery chargers designed for desulfating lead-acid batteries are commercially available. A battery will be unrecoverable if the active material has been lost from the plates, or if the plates are bent due to over temperature or over charging.
Batteries which have sat unused for long periods of time can be prime candidates for desulfation. A long period of self-discharge allows the sulfate crystals to form and become very large. Some typical cases where lead acid batteries are not used frequently enough are planes, boats (esp sail boats), old cars, and home power systems with battery banks that are under utilized.
Some charging techniques can aid in prevention such as equalization charging and cycles through discharging and charging regularly. It is recommended to follow battery manufacturer instructions for proper charging.
SLI batteries (starting, lighting, ignition; i.e. car batteries) suffer most deterioration because vehicles normally stand unused for relatively long periods of time. Deep cycle and motive power batteries are subjected to regular controlled overcharging, hence eventually succumb to corrosion of the positive plate grids, not to sulfation.
Extreme weather can also cause sulfation in batteries. Extreme heat in the summer increases the amount of sulfates that come from batteries. Electronic components putting a constant drain on a battery also increase the amount of sulfation. Keeping a battery in a cool location and keeping it charged help prevent this.
A typical lead–acid battery contains a mixture with varying concentrations of water and acid. Sulfuric acid has a higher density than water, which causes the acid formed at the plates during charging to flow downward and collect at the bottom of the battery. Eventually the mixture will again reach uniform composition by diffusion, but this is a very slow process. Repeated cycles of partial charging and discharging will increase stratification of the electrolyte, reducing the capacity and performance of the battery because the lack of acid on top limits plate activation. The stratification also promotes corrosion on the upper half of the plates and sulfonation at the bottom.
Periodic overcharging creates gaseous reaction products at the plate, causing convection currents which mix the electrolyte and resolve the stratification. Mechanical stirring of the electrolyte would have the same effect. Batteries in moving vehicles are also subject to sloshing and splashing in the cells, as the vehicle accelerates, brakes, and turns.
Risk of explosion
Excessive charging electrolyzes some of the water, emitting hydrogen and oxygen. This process is known as "gassing". Wet cells have open vents to release any gas produced, and VRLA batteries rely on valves fitted to each cell. Catalytic caps are available for flooded cells to recombine hydrogen and oxygen. A VRLA cell normally recombines any hydrogen and oxygen produced inside the cell, but malfunction or overheating may cause gas to build up. If this happens (for example, on overcharging) the valve vents the gas and normalizes the pressure, producing a characteristic acid smell. Valves can sometimes fail however, if dirt and debris accumulate, allowing pressure to build up.
Accumulated hydrogen and oxygen sometimes ignites in an internal explosion. The force can burst the plastic casing or blow the top off the battery, spraying acid and casing fragments. An explosion in one cell may ignite the combustible gas mixture in remaining cells. In a poorly ventilated area, connecting or disconnecting a closed circuit (such as a load or an operating charger) directly to the battery terminals may cause an explosion as the sparks produced ignite the gas mixtures vented from the cells.
The cell walls of VRLA batteries typically swell when the internal pressure rises. The deformation varies from cell to cell, and is greater at the ends where the walls are unsupported by other cells. Such over-pressurized batteries should be carefully isolated and discarded. Personnel working near batteries at risk for explosion should protect their eyes and exposed skin from burns due to spraying acid and fire by wearing a face shield, overalls, and gloves. Using goggles instead of a face shield sacrifices safety by leaving one's face exposed to acid and heat from a potential explosion.
According to a 2003 report entitled "Getting the Lead Out", by Environmental Defense and the Ecology Center of Ann Arbor, Mich., the batteries of vehicles on the road contained an estimated Script error: No such module "convert". of lead. Some lead compounds are extremely toxic. Long-term exposure to even tiny amounts of these compounds can cause brain and kidney damage, hearing impairment, and learning problems in children. The auto industry uses over Script error: No such module "convert". every year, with 90% going to conventional lead-acid vehicle batteries. While lead recycling is a well-established industry, more than Script error: No such module "convert". ends up in landfills every year. According to the federal Toxic Release Inventory, another Script error: No such module "convert". are released in the lead mining and manufacturing process.
Attempts are being made to develop alternatives (particularly for automotive use) because of concerns about the environmental consequences of improper disposal and of lead smelting operations, among other reasons. Alternatives are unlikely to displace them for applications such as engine starting or backup power systems, since the batteries are low-cost although heavy.
Lead–acid battery recycling is one of the most successful recycling programs in the world. In the United States 99% of all battery lead was recycled between 2009 and 2013. An effective pollution control system is a necessity to prevent lead emission. Continuous improvement in battery recycling plants and furnace designs is required to keep pace with emission standards for lead smelters.
Chemical additives have been used ever since the lead-acid battery became a commercial item, to reduce lead sulfate build up on plates and improve battery condition when added to the electrolyte of a vented lead–acid battery. Such treatments are rarely, if ever, effective.
Two compounds used for such purposes are Epsom salts and EDTA. Epsom salts reduces the internal resistance in a weak or damaged battery and may allow a small amount of extended life. EDTA can be used to dissolve the sulfate deposits of heavily discharged plates. However, the dissolved material is then no longer available to participate in the normal charge/discharge cycle, so a battery temporarily revived with EDTA will have a reduced life expectancy. Residual EDTA in the lead–acid cell forms organic acids which will accelerate corrosion of the lead plates and internal connectors.
The active materials change physical form during charge/discharge, resulting in growth and distortion of the electrodes, and shedding of electrode into the electrolyte. Once the active material has fallen out of the plates, it cannot be restored into position by any chemical treatment. Similarly, internal physical problems such as cracked plates, corroded connectors, or damaged separators cannot be restored chemically.
Corrosion of the external metal parts of the lead–acid battery results from a chemical reaction of the battery terminals, lugs and connectors.
Corrosion on the positive terminal is caused by electrolysis, due to a mismatch of metal alloys used in the manufacture of the battery terminal and cable connector. White corrosion is usually lead or zinc sulfate crystals. Aluminum connectors corrode to aluminum sulfate. Copper connectors produce blue and white corrosion crystals. Corrosion of a battery's terminals can be reduced by coating the terminals with petroleum jelly or a commercially available product made for the purpose.
If the battery is over-filled with water and electrolyte, thermal expansion can force some of the liquid out of the battery vents onto the top of the battery. This solution can then react with the lead and other metals in the battery connector and cause corrosion.
The electrolyte can weep from the plastic-to-lead seal where the battery terminals penetrate the plastic case.
Acid fumes that vaporize through the vent caps, often caused by overcharging, and insufficient battery box ventilation can allow the sulfuric acid fumes to build up and react with the exposed metals.
With the broad range of possible electrical attributes, a part number nomenclature is used by many battery manufacturers to convey basic information such as voltage, ampere-hour capacity, and terminals. The format follows a pattern such as <mfg><voltage><capacity>.
|Part Number||Manufacturer||Voltage, V||Capacity, A·h|
|SP12-18HR||Sigmas Battery Tek||12||18.0|
|UB12180||Universal Power Group||12||18.0|
Some vendors append a suffix, indicating the terminal types, terminal locations, and battery dimensions. Batteries for passenger motor vehicles usually use BCI sizing nomenclature.
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These data suggest that certain subtle neurobehavioral effects in children may occur at very low PbBs. (PbB means lead blood level)
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- Lead Acid Battery Desulfator (Home Power #77 June/July 2000) 
- Battery Plate Sulfation (MagnaLabs)
- Battery Desulfation 
- Lead Acid Batteries 
- DC Supply! (April 2002) 
- Some Technical Details on Lead Acid Batteries 
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- Battery Council International (BCI), lead–acid battery manufacturers' trade organization.
- Car and Deep Cycle Battery Frequently Asked Questions
- ToxFAQs: Lead
- National Pollutant Inventory – Lead and Lead Compounds Fact Sheet
- Case Studies in Environmental Medicine – Lead Toxicity
- Lead Acid Battery Desulfator (Home Power #77 June/July 2000)
- Battery Plate Sulfation (MagnaLabs)
- Battery Desulfation
- Technical Details on lead–acid Batteries
- Primer On Lead–Acid Storage Batteries at the U.S. Department of Energy.
- Battery dischargers - Description and treatment of sulphated batteries