Lead Acid Batteries
Characteristics
Lead acid batteries were invented in 1859 by Gaston Planté and first demonstrated to the French Academy of
Sciences in 1860. They remain the technology of choice for automotive SLI (Starting, Lighting and Ignition)
applications because they are robust, tolerant to abuse, tried and tested and because of their low cost. For higher
power applications with intermittent loads however, Lead acid batteries are generally too big and heavy and they
suffer from a shorter cycle life and typical usable power down to only 50% Depth of Discharge (DOD). Despite these
shortcomings Lead acid batteries are still being specified for PowerNet applications (36 Volts 2 kWh capacity)
because of the cost, but this is probably the limit of their applicability and NiMH and Li-Ion batteries are making
inroads into this market. For higher voltages and cyclic loads other technologies are being explored.
Lead-acid batteries are composed of a Lead-dioxide cathode, a sponge metallic Lead anode and a Sulphuric acid
solution electrolyte. This heavy metal element makes them toxic and improper disposal can be hazardous to the
environment.
The cell voltage is 2 Volts
Discharge
During discharge, the lead dioxide (positive plate) and lead (negative plate) react with the electrolyte of sulfuric acid
to create lead sulfate, water and energy.
Charge
During charging, the cycle is reversed: the lead sulfate and water are electro-chemically converted to lead, lead oxide
and sulfuric acid by an external electrical charging source.
Many new competitive cell chemistries are being developed to meet the requirements of the auto industry for EV and
HEV applications.
Even after 150 years since its invention, improvements are still being made to the lead acid battery and despite its
shortcomings and the competition from newer cell chemistries the lead acid battery still retains the lion's share of the
high power battery market.
Advantages
Low cost.
Reliable. Over 140 years of development.
Robust. Tolerant to abuse.
Tolerant to overcharging.
Low internal impedance.
Can deliver very high currents.
Indefinite shelf life if stored without electrolyte.
Can be left on trickle or float charge for prolonged periods.
Wide range of sizes and capacities available.
Many suppliers world wide.
The world's most recycled product.
Shortcomings
Very heavy and bulky.
Typical coulombic charge efficiency only 70% but can be as high as 85% to 90% for special designs.
Danger of overheating during charging
Not suitable for fast charging
Typical cycle life 300 to 500 cycles .
Must be stored in a charged state once the electrolyte has been introduced to avoid deterioration of the active
chemicals.
Gassing is the production and release of bubbles of hydrogen and oxygen in the electrolyte during the charging
process, particularly due to excessive charging, causing loss of electrolyte. In large battery installations this can
cause an explosive atmosphere in the battery room. Sealed batteries are designed to retain and recombine these
gases. (See VRLA below)
Sulphation may occur if a battery is stored for prolonged periods in a completely discharged state or very low state
of charge, or if it is never fully charged, or if electrolyte has become abnormally low due to excessive water loss from
overcharging and/or evaporation. Sulphation is the increase in internal resistance of the battery due to the formation
of large lead sulphate crystals which are not readily reconverted back to lead, lead dioxide and sulphuric acid during
re-charging. In extreme cases the large crystals may cause distortion and shorting of the plates. Sometimes
sulphation can be corrected by charging very slowly (at low current) at a higher than normal voltage.
Completely discharging the battery may cause irreparable damage.
Shedding or loss of material from the plates may occur due to excessive charge rates or excessive cycling. The
result is chunks of lead on the bottom of the cell, and actual holes in the plates for which there is no cure. This is
more likely to occur in SLI batteries whose plates are composed of a Lead "sponge", similar in appearance to a very
fine foam sponge. This gives a very large surface area enabling high power handling, but if deep cycled, this sponge
will quickly be consumed and fall to the bottom of the cells.
Toxic chemicals
Very heavy and bulky
Lower temperature limit -15 °C
Decomposition of the Electrolyte Cells with gelled electrolyte are prone to deterioration of the electrolyte and
unexpected failure. Such cells are commonly used for emergency applications such as UPS back up in case of loss
of mains power. So as not to be caught unawares by an unreliable battery in an emergency situation, it is advisable to
incorporate some form of regular self test into the battery.
Charging
Charge immediately after use.
Lasts longer with partial discharges.
Charging method: constant voltage followed by float charge.
Fast charge not possible but charging time can be reduced using the V Taper charge control method.
Applications
Automotive and traction applications.
Standby/Back-up/Emergency power for electrical installations.
Submarines
UPS (Uninterruptible Power Supplies)
Lighting
High current drain applications.
Sealed battery types available for use in portable equipment.
Costs
Low cost
Flooded lead acid cells are one of the least expensive sources of battery power available.
Deep cycle cells may cost up to double the price of the equivalent flooded cells.
Varieties of Lead Acid Batteries
Lead Calcium Batteries
Lead acid batteries with electrodes modified by the addition of Calcium providing the following advantages:
More resistant to corrosion, overcharging, gassing, water usage, and self-discharge, all of which shorten battery
life.
Larger electrolyte reserve area above the plates.
Higher Cold Cranking Amp ratings.
Little or No maintenance.
Lead Antimony Batteries
Lead acid batteries with electrodes modified by the addition of Antimony providing the following advantages:
Improved mechanical strength of electrodes - important for EV and deep discharge applications
Reduced internal heat and water loss.
Longer service life than Calcium batteries.
Easier to recharge when completely discharged.
Lower cost.
Lead Antimony batteries have a higher self discharge rate of 2% to 10% per week compared with the 1% to 5% per
month for Lead Calcium batteries.
Valve Regulated Lead Acid (VRLA) Batteries
Also called Sealed Lead Acid (SLA) batteries.
This construction is designed to prevent electrolyte loss through evaporation, spillage and gassing and this in turn
prolongs the life of the battery and eases maintenance. Instead of simple vent caps on the cells to let gas escape,
VRLA have pressure valves that open only under extreme conditions. Valve-regulated batteries also need an
electrolyte design that reduces gassing by impeding the release to the atmosphere of the oxygen and hydrogen
generated by the galvanic action of the battery during charging. This usually involves a catalyst that causes the
hydrogen and oxygen to recombine into water and is called a recombinant system. Because spillage of the acid
electrolyte is eliminated the batteries are also safer.
AGM Absorbed Glass Mat Battery
Also known as Absorptive Glass Micro-Fibre
Used in VRLA batteries the Boron Silicate fibreglass mat which acts as the separator between the electrodes and
absorbs the free electrolyte acting like a sponge. Its purpose is to promote recombination of the hydrogen and oxygen
given off during the charging process. No silica gel is necessary. The fibreglass matt absorbs and immobilises the
acid in the matt but keeps it in a liquid rather than a gel form. In this way the acid is more readily available to the
plates allowing faster reactions between the acid and the plate material allowing higher charge/discharge rates as
well as deep cycling.
This construction is very robust and able to withstand severe shock and vibration and the cells will not leak even if the
case is cracked.
AGM batteries are also sometimes called "starved electrolyte" or "dry", because the fibreglass mat is only 95%
saturated with Sulfuric acid and there is no excess liquid.
Nearly all AGM batteries are sealed valve regulated "VRLA".
AGM's have a very low self-discharge rate of from 1% to 3% per month
Gel Cell
This is an alternative recombinant technology to also used in VRLA batteries to promote recombination of the gases
produced during charging. It also reduces the possibility of spillage of the electrolyte. Prone to damage if gassing is
allowed to occur, hence charging rates may be limited. They must be charged at a slower rate (C/20) to prevent
excess gas from damaging the cells. They cannot be fast charged on a conventional automotive charger or they may
be permanently damaged.
Used for UPS applications.
SLI Batteries (Starting Lighting and Ignition)
This is the typical automotive battery application. Automotive batteries are designed to be fully charged when starting
the car; after starting the vehicle, the lost charge, typically 2% to 5% of the charge, is replaced by the alternator and
the battery remains fully charged. These batteries are not designed to be discharged below 50% Depth of Discharge
(DOD) and discharging below these levels can damage the plates and shorten battery life.
Deep Cycle Batteries
Marine applications, golf buggies, fork lift trucks and electric vehicles use deep cycle batteries which are designed to
be completely discharged before recharging. Because charging causes excessive heat which can warp the plates,
thicker and stronger or solid plate grids are used for deep cycling applications. Normal automotive batteries are not
designed for repeated deep cycling and use thinner plates with a greater surface area to achieve high current
carrying capacity.
Automotive batteries will generally fail after 30-150 deep cycles if deep cycled, while they may last for thousands of
cycles in normal starting use (2-5% discharge).
If batteries designed for deep cycling are used for automotive applications they must be "oversized" by about 20% to
compensate for their lower current carrying capacity.
1.0 Types of Lead-Acid Batteries
We are dependent on lead-acid batteries for many uses in our lives that can be subdivided into
three broad categories: engine-starting, motive power and standby power.
1.1 Engine Starting
The most common use of engine-starting batteries is in automobiles and trucks. They provide
energy for starting, lighting and fuel ignition. Other uses occur in lawn mowers, snowmobiles,
boats and all-terrain vehicles. The features of the battery for these applications are, for example:
 discharge at very high rates and at temperatures ranging from –20ºF to 200ºF
 a sufficient reserve capacity to operate vehicle electrical systems when charger fails and
to power off-key drains
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thousands of engine starts (resulting in shallow discharge/charge cycles)
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low self-discharge rates so they operate after long periods of non-use
 charging by an alternator
 three-to-five-year life.
An engine-starting battery must be capable of high bursts of power. All types of polarization
must be minimized to produce the highest voltage possible. Therefore, engine-starting batteries
are designed with a very large area of working electrode surface. This reduces current density,
which in turn reduces activation polarization during discharge. Large surface area is achieved by
designing batteries with large numbers of very thin (as thin as 1 millimeter) electrodes. Resistive
polarization is minimized by reducing the spacing between plates, reducing metallic conductive
paths and using heavy-duty plate-connecting straps. These design features also reduce
concentration polarization since the diffusion gradient from the bulk electrode to the reacting
surface is reduced.
To provide reserve power in case of charger failure and to operate electrical components with
the engine off, the engine-starting battery must contain sufficient active material (lead dioxide,
lead and sulfuric acid) in the plates. This necessitates a design compromise requiring the use of
thicker plates and a larger amount of electrolyte. The result is an increase in polarization and a
reduction in power. Design invariably involves a trade-off between the highest cranking power
and adequate reserve capacity.
1.2 Motive Power
The function of this type of battery, also called traction battery, is to propel an electric vehicle
(EV). EVs are widely used in the material handling industry for supplying energy to fork lift
trucks. Other uses include electric golf cars, mining vehicles, airport baggage handling tugs,
sweepers/scrubbers and wheelchairs. These applications require the battery to be capable of:
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moderate discharge rates (three to six hours of discharge before recharging)
high capacities up to thousands of ampere-hours
cycling at high depths of discharge (over 80% of capacity removed before recharging)
operation over a wide range of temperatures from 0ºF to 100ºF
controlled charging
five-year life (approximately 1500 cycles).
The principal requirements of traction batteries are high capacity and long life. Since capacity
is the fuel that powers an EV, the higher the capacity, the more work that can be done before the
need to recharge. For greater capacity, a battery is designed with thick electrodes containing
large amounts of active material. To obtain increased active material, the density is increased to a
higher level than that used in automotive batteries. Electrodes are considerably thicker than those
in automotive batteries. Typical motive-power plates can be 6–7 millimeters thick. Thick
electrodes give batteries longer life. Since the principal mechanism that causes these batteries to
wear out is grid corrosion, using thick grids also extends life. Since discharge rates are moderate
to low and the electrodes are made from thick lead-alloy grids, activation and resistive
polarization are not major components of the voltage drop during discharge. This is primarily
caused by concentration polarization due to the increased distance for ionic migration and
diffusion. However, remember that during deep discharge, the amount of lead sulfate in the
electrodes is increased significantly, causing an increase in resistance as discharge progresses.
Eventually, active materials in the plates or sulfuric acid are consumed, causing a rapid increase
in polarization and a reduction in the voltage at the end of discharge.
The battery is both the energy source and the “fuel.” Thus, it is important to know the depth
of discharge or how much “fuel” remains. Since the life of a motive-power battery is
proportional to depth of discharge, knowing how much capacity has been removed permits
recharging before damaging the battery by over discharging.
1.3 Standby Power
Most of us never see a standby-power battery, though they are a major segment of the battery
industry. Their use is growing at a faster rate than engine-starting and motive-power batteries.
They are mostly used as components in larger systems and housed away from public view in
dedicated rooms and cabinets. They function to provide energy when the main power is
interrupted, i.e., during power outages. For example, we take for granted that our telephones
operate during a blackout. In the United States and in other developed countries, entire telephone
systems are supported by batteries that can supply power for up to several hours. Standby power
permits making emergency calls in spite of power outages. The principal areas where standbypower batteries are used are:
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telecommunications
uninterruptible power systems (UPS)
switchgear and control operation
emergency lighting
security.
Of these, telecommunications and uninterruptible power systems are the largest segments in
the United States with 48% ($310 million) and 27% ($174 million), respectively, of the total
standby-power battery sales (1998 figures).
Each of these applications has different electrical requirements which, in turn, require
different battery designs. However, they have in common the following features:
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wide range of discharge rates
shallow cycling except in emergency situations
narrow temperature range of operation
float charging
up to 20-year-life expectancy.
Standby batteries spend most of their life being charged at a rate that is just sufficient to
maintain the battery in a state of full charge. This is known as float charging. It is also important
that these batteries have low gassing rates during float charging so that water loss is minimized.
A low gassing rate is achieved by controlling the polarization of the positive and negative
electrodes to a value just sufficient to maintain full charge while at the same time minimizing
electrolysis of water. The gassing rate is also reduced by the use of calcium, calcium/tin or low
antimony-alloy grids. These alloys have low corrosion rates, thereby prolonging battery life.
The lead-calcium alloy grids in the positive plates slowly degrade by intergranular corrosion.
In this process, corrosion takes place between the metallic grains and produces corrosion
products that progressively push grains apart. This causes the alloy to expand and the plates to
grow larger with time. To allow for this, the battery designer suspends the positive plates on a
plastic bar attached to the negative plates, thereby providing space for the positive plate to
expand. This design feature allows the grid to expand without placing strain on the cover seal.
Another feature of standby batteries is the use of flame-retardant vent plugs. These are
necessary to prevent sparks or arcs from outside the battery communicating with the gases inside
the cells and causing an explosion.
1.4 Valve-Regulated Lead-Acid Batteries
As we have already seen, conventional lead-acid batteries evolve hydrogen and oxygen when
they are charged. These gases are a result of the electrolysis of water inside the battery and,
therefore, water is consumed and must be replaced. Water replacement must be carried out
frequently enough to avoid drying out the battery, a job that comprises a major activity of battery
maintenance. Also, these gases are an explosion hazard and carry with them a fine mist of
sulfuric acid that can be deposited on external conductors, resulting in corrosion. Since oxygen
and hydrogen gases emitted from cells can form explosive mixtures, the battery room or
enclosure has to be ventilated.
For many years, the battery industry has pursued a battery design that eliminates this
maintenance and reduces the work and cost of owning batteries. The primary objective has been
to develop a way to recombine the hydrogen and oxygen back into water inside the battery,
eliminating the need to add water. The objective has been achieved and batteries that employ
recombination principles are now widely available.
This new lead-acid battery design is the valve-regulated or recombination-type battery.
Operating on the gas-recombining principle, it gets its name because it is fitted with a pressurerelease valve that maintains a certain oxygen pressure inside the battery. In these batteries,
oxygen generated inside the cell during charging is recombined within the cell to re-form water.
1.4.1 Principle of Operation
Normally, when a lead-acid battery is overcharged, oxygen is evolved at the positive plate and
hydrogen is evolved at the negative plate. These gases are vented from the battery and the water
that is consumed in producing them is replenished periodically from an outside source during
normal maintenance. In the valve-regulated battery, a method has been found to recombine the
gases inside the cell, thereby avoiding gas emission and the need to add water during the life of
the battery.
Some years ago, it was discovered that if oxygen gas diffused to the negative plate, it
would react with the negative sponge lead and be consumed. However, the amount of oxygen
that could effectively reach the negative plate was severely restricted by the separators and the
electrolyte. These formed a barrier to the diffusion of oxygen so that it was easier for the gas to
escape from the cell than to migrate to the negative plate. With recently discovered and instituted
design changes that promote diffusion of oxygen, virtually all of it can reach the negative plate
and be recombined to water.
Oxygen will react at the negative plate in the presence of sulfuric acid as quickly as it can
diffuse to the lead surface according to the following reaction:
Pb + H2SO4 + ½O2 = PbSO4 + H2O
Thus, the oxygen that diffuses to the negative is converted to water. As a result of this
reaction, no water is emitted from the cell and, therefore, no water needs to be added. For this
reason, these batteries are sometimes referred to as “maintenance-free,” although other forms of
routine maintenance are still required.
There are two distinct designs of recombination battery currently in use: absorbed electrolyte
and gelled electrolyte.
1.4.2 Absorbed Electrolyte
The separator is replaced by a layer of porous glass mat. The cells are sealed with a valve to
keep the cell pressurized at 2–5 pounds per square inch. The cell is filled with just enough
electrolyte to wet the plates and partially wet the separator, thus creating an electrolyte-starved
condition. Because the separator is not completely saturated with electrolyte, oxygen gas
generated at the positive plate can diffuse through it and migrate to the negative plate. The
pressure valve keeps the gas inside the cell long enough for diffusion to take place. As the
oxygen is reduced at the negative plate, the negative-plate lead is oxidized to lead sulfate. This
prevents the negative plate from becoming fully charged. Therefore, it does not start to evolve
hydrogen. In some designs, an excess of negative active material is included to ensure the
negative plate does not become fully charged. This provides additional protection against
hydrogen evolution. Since the container of the cell or battery is held under pressure, it must be
made of a material that will not distort.
1.4.3 Gelled Electrolyte
In the gelled electrolyte design, the plates are separated by conventional separators. The cell is
filled with a gel composed of sulfuric acid and silica. After the gel is added to the cell, it hardens
in a manner similar to gelatin so that it is immobilized. In time, the gel gradually dries out,
creating very small cracks and fissures. These act as channels for oxygen to diffuse from the
positive plate to the negative electrode.
Though the way in which transport of oxygen to the negative plate is different between the
absorbed and gelled electrolyte designs, the principle of operation is the same for both. With the
exception of the separator and gel, both types of batteries are constructed the same way.
1.4.4 Construction of Valve-Regulated Batteries
Grids
The recombination reaction in a valve-regulated battery is very sensitive to poisoning by low
levels of impurities in the grid, active materials and electrolyte. For this reason, it is important
that the alloys used to make the grids contain very low levels of metallic impurities. Virtually all
valve-regulated batteries have positive grids made from either lead-calcium-tin alloys or pure
lead while the negative grids are cast from lead-calcium alloy.
Plates
The plates are made in a manner similar to conventional automotive and industrial batteries.
They are coated with a paste made from leady oxide, water and sulfuric acid. Like conventional
batteries, the negative plates contain expander. After pasting, the plates are cured and dried using
methods yet to be described. Since the amount of compression of the glass mat in an absorbed
electrolyte design is very important to achieve exactly the right amount of wetting, it is important
also to control the thickness of the plates very carefully.
Assembly
The assembly methods of valve-regulated batteries differ substantially from those of
automotive and industrial batteries. In the absorbed-electrolyte type, the plates are clad in glass
felt which acts both as separator and electrolyte absorber. This glass material is highly porous
and has the ability to absorb a considerable amount of sulfuric acid. The amount of acid the glass
absorbs can be adjusted by altering its compression – the greater the compression, the less the
electrolyte absorbed. The correct compression is important to ensure that the cell contains
enough electrolyte to sustain the discharge reaction of the battery while making sure that it is not
saturated with acid. Mat oversaturation can prevent the diffusion of the oxygen gas from the
positive plate to the negative plate.
In the absorbed-electrolyte design, the glass-clad plates are stacked to produce the required
capacity and then the element is compressed so that it can be inserted into the container. The
cover is cemented or welded in place and the electrolyte is then added. The assembled cell or
battery can then be formed in a manner similar to that used for motive-power or standby batteries.
In the gelled-electrolyte type, the cell element is stacked in the standard manner with
separators between the positive and negative plates. It is then inserted into the container and the
cover is installed. The cell then can be filled with either gelled or liquid electrolyte. If liquid
electrolyte is used, the cell is formed, drained and then refilled with gel. Alternatively, the
battery can be filled with gelled sulfuric acid and then formed. Cell formation is done more
quickly with the liquid electrolyte process than with the gel-filled. However, it does involve the
extra step and added cost.
1.4.5 The Advantages of Valve-Regulated Batteries
Reduced Maintenance
Without the need to add water, most routine maintenance required on flooded batteries is
eliminated. Periodic cleaning and servicing are greatly reduced due to the elimination of water
spills on top of the battery and the associated corrosion of terminals.
Elimination of Gassing
No gases are evolved from the battery because they are recombined inside the container. Thus,
there is no need to ventilate the battery compartment or room. These batteries are intrinsically
safer than flooded types. The acidic spray emitted from vents of flooded batteries during
charging that can cause corrosion of battery terminals and affect adjacent electronic circuitry
does not occur.
Operation in Any Orientation
Electrolyte immobilization prevents acid spills. These batteries can be used on their side, a big
advantage in installations where space is limited.
1.4.6 The Limitations of Valve-Regulated Batteries
Cycle Life
At the time of this writing, valve-regulated batteries have not been developed for applications
requiring a large number of deep cycles. With deep-discharge cycling, these batteries display a
rapid decline in capacity, known as premature capacity loss (PCL). Two reasons for PCL are
proffered:
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formation of a barrier layer of corrosion products between the active material and the grid that
cannot be removed by charging. The layer progressively and eventually builds up enough to
reduce capacity below a useful level;
loss of cohesion of the active material that gradually reduces inter-particle contact and thus
capacity. Recent work has shown that increased compression on the electrodes reduces the
effect of these structural changes, slowing down the capacity loss during cycling.
Another interesting finding, though not fully understood, is that rapid recharging, maybe
resulting in better preservation of the structure of the active material, considerably reduces the
capacity loss.
Unfortunately, lead-antimony alloys, which are widely used in batteries designed for cycling,
cannot be used in valve-regulated batteries. They release antimony ions into the electrolyte that
deposit on the negative electrode and increase hydrogen evolution.
These batteries do, however, have better cycle life than conventional flooded lead-calcium
batteries. Therefore, they can be used satisfactorily in limited-cycle-life applications.
Float Life
Though these batteries are often used in float applications, they do not match the 20-year life
of flooded types. While still in dispute, a ten-year life is likely the maximum achievable for
valve-regulated batteries. A major reason for this is the difficulty in maintaining the proper
degree of polarization of the electrodes during float charging. When the cell is charged, the
current that would normally polarize the negative electrode and keep it charged is fully
consumed in reducing the oxygen that has migrated from the positive electrode. Consequently,
the negative electrode is depolarized and will be at its open circuit potential. Under this condition,
it gradually self-discharges due to local action and loses capacity. However, if the float current is
increased to assure that the negative remains fully charged, it will produce hydrogen gas that will
cause loss of water and eventual failure by drying out. This “knife edge” situation makes it
almost impossible to control the polarization of both electrodes to the correct degree during float
charging. Impurities in the active material, grids and electrolyte can also affect the polarization
of the negative electrode and either increase hydrogen evolution or self-discharge. Work is under
way to develop charging strategies to overcome the problem. Catalysts are being looked at that
would recombine hydrogen and oxygen, thus allowing the negative to be adequately polarized
without risking dry-out.
1.4.7 Applications of Valve-Regulated Batteries
Valve-regulated batteries are becoming more widely used for standby applications and are
finding limited use in cycling applications such as motive power. The major standby uses include:
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telephone systems
uninterruptible power systems
burglar and fire alarms
emergency lighting.
The major attraction is reduced maintenance for the user.
The most popular motive-power application, at present, is for wheelchairs. The spill-proof
properties and lack of gassing are the most attractive features.