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Subj: Lead-Acid Batteries
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Lead-acid batteries, invented in 1859 by French physicist Gaston
Plant, are the oldest type of rechargeable battery. Despite having the
second lowest energy-to-weight ratio (next to the nickel-iron battery and a
correspondingly low energy-to-volume ratio, their ability to supply high
surge currents means that the cells maintain a relatively large
power-to-weight ratio. These features, along with their low cost, make them
attractive for use in cars, to provide the high current required by automobile
starter motors.
Contents:
1) Electrochemistry
2) Construction of battery
2.1) Plates
2.2) Separators
3) Classification of lead acid batteries
3.1) By production technology
3.2) By application
4) Applications
5) Cycles
5.1) Starting batterie
5.2) Deep cycle batteries
5.3) Fast and slow charge and discharge
6) Valve regulated lead acid batteries
7) Exploding batteries
8) Environmental concerns
9) Additives
10) Maintenance precautions
1) Electrochemistry
Each cell contains (in the charged state) electrodes of lead metal
(Pb) and lead (IV) dioxide (PbO2) in an electrolyte of about 33.5% w/w
(6 Molar) sulfuric acid (H2SO4). In the discharged state both electrodes turn
into lead(II) sulfate (PbSO4) and the electrolyte loses its dissolved sulfuric
acid and becomes primarily water . Due to the freezing-point depression of
water, as the battery discharges and the concentration of sulfuric acid
decreases, the electrolyte is more likely to freeze.
The chemical reactions are (charged to discharged):
Anode (oxidation):
+1 -1 -
PbSO + 5H O <-> PbO + 3H O + HSO + 2e e= 1.685V
4 2 2 3 4
Cathode (reduction):
+1 -1 -1
PbSO + H O + 2e <-> Pb + HSO + H O e= .356V
4 3 4 2
Because of the open cells with liquid electrolyte in most lead-acid
batteries, overcharging with excessive charging voltages will generate oxygen
and hydrogen gas by electrolysis of water, forming an explosive mix. This
should be avoided. Caution must also be observed because of the extremely
corrosive nature of sulfuric acid.
Practical cells are usually not made with pure lead but have small
amounts of antimony , tin , or calcium alloyed in the plate material.
These are general voltage ranges for six-cell lead-acid batteries:
* Open-circuit (quiescent) at full charge: 12.6 V to 12.8 V.
* Open-circuit at full discharge: 11.8 V to 12.0 V
* Loaded at full discharge: 10.5 V.
* Continuous-preservation (float) charging: 13.4 V for gelled electrolyte;
13.5 V for AGM (absorbed glass mat) and 13.8 V for flooded cells
All voltages are at 20 ๘C, and must be adjusted -0.022V/๘C for
temperature changes.
Float voltage recommendations vary, according to the manufacturer's
recommendation.
Precise (๑0.05 V) float voltage is critical to longevity; too low
(sulfation) is almost as bad as too high (corrosion and electrolyte loss)
Typical (daily) charging: 14.2 V to 14.5 V (depending on
manufacturer's recommendation)
Equalization charging (for flooded lead acids): 15 V for no more than
2 hours. Battery temperature must be monitored.
Gassing threshold: 14.4 V
After full charge the terminal voltage will drop quickly to 13.2 V
and then slowly to 12.6 V.
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. falling as the battery discharges.
Some battery designs, such as those used in electronic flash units, have a
imple hydrometer built in using coloured 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 apprise the commander as to
how much underwater endurance the boat had remaining.
2) Construction of battery
2.1) Plates
The principle of the lead acid cell can be demonstrated with simple
sheet lead plates for the two electrodes. However such a construction would
only produce around an amp for roughly postcard sized plates, and it would
not produce such a current for more than a few minutes.
Gaston Plant realised that a plate construction was required that
gave a much larger effective surface area. Plant's method of producing the
plates has been largely unchanged and is still used in stationary
applications.
The Faure pasted-plate construction is typical of automotive
batteries. Each 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 mixture of red lead and 33% dilute sulfuric acid.
(Different manufacturers have modified the mixture). The paste is pressed
into the holes in the plates which are slightly tapered on both sides to
assist in retention of the paste. This porous paste allows the acid to react
with the lead inside the plate, increasing the surface area many fold. At
this stage the positive and negative plates are similar, however expanders
and additives vary their internal chemistry to assist in operation when in
use. Once dry, the plates are then stacked together 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 together. After the acid has been added to the cell, the cell is
given its first forming charge. The positive plates gradually turn the
chocolate brown colour of lead dioxide, and the negative turn the slate gray
of 'spongy' lead. Such a cell is ready to be used.
One of the problems with the plates in a lead-acid battery is that
the plates change size as the battery charges and discharges, the plates
increasing in size as the active material absorbs sulfate> from the acid
during discharge, and decreasing as they give up the sulfate during charging.
This causes the plates to gradually shed the paste during their life. It is
important that there is plenty of room underneath the plates to catch this
shed material. If this material reaches the plates a shorted cell will occur.
2.2) Separators
Separators are used between the positive and negative plates of a
lead acid battery to 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.
Various materials have been used to make separators:
* wood
* rubber
* glass fiber mat
* cellulose
* sintered PVC
* microporous PVC/polyethylene.
An effective separator must possess a number of mechanical properties;
applicable considerations include 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 operating temperature range of the
battery.
In the battery service condition the following reaction can be shown :
PbO2 + 2H+ + SO4-2 = PbSO4 + H2O + ซ O2
PbO2 + (oxidizable separator material) + H2SO4 = PbSO4 + (oxidized material)
3)Classification of lead acid batteries
3.1) By production technology
Flooded/Wet cell batteries;
Valve Regulated Lead Acid batteries;
AGM: Absorbed Glass Mat batteries;
Gel cell batteries ;
Carbon Foam Lead Acid batteries.
3.2) By application
Stand-by (stationary) batteries;
Motor vehicle starting, lighting and ignition (SLI) batteries ;
Traction (propulsion) batteries.
4) Applications
Wet cells designed for deep discharge are commonly used in golf carts
and other battery electric vehicles, large backup power supplies for telephone
and computer centers and off-grid household electric power systems.
Gel batteries are used in back-up power supplies for alarm and
smaller computer systems (particularly in uninterruptible power supplies) and
for electric scooters, electrified bicycles and marine applications. Unlike
wet cells, gel cells are sealed, with pressure relief valves in case of
overcharging. In normal use they cannot spill liquid electrolyte.
Absorbed glass mat (AGM) cells are also sealed and used in battery
electric vehicles, as well as applications where there is a fairly high risk
of the battery being laid on its side or over-turned, such as motorcycles.
Historically, lead-acid batteries were used to supply the filament
(heater) voltage (usually between 2 and 12 volts with 6 V being most common)
in vacuum tube (valve) radio receivers in areas where no mains electricity
supply was available. Such radios typically used two batteries: a lead-acid
"A" battery for the filament voltage and a higher voltage (45 V-120 V) "dry"
non-rechargeable "B" battery for the plate (anode) voltage.
Lead-acid batteries are generally used in emergency lighting in case
of power failure.
They are also used in vehicles such as forklifts, in which the low
energy-to-weight ratio may in fact be considered a benefit since the battery
can be used as a counterweight. Large arrays of lead-acid cells are used as
standby power sources for telecommunications facilities, generating stations,
and computer data centers. Large lead-acid batteries are also used to power
the electric motors in diesel-electric (conventional) submarines and are used
on nuclear submarines as well.
5) Cycles
5.1) Starting batteries
Lead acid batteries designed for starting automotive engines aren't
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. A common
misconception is that starting batteries should always be kept on float
charge. In reality, this practice will encourage corrosion in the electrodes
and result in premature failure. Starting batteries should be kept
open-circuit but charged regularly (at least once every two weeks) to prevent
sulfation.
5.2) 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.
Marine/Motorhome batteries, sometimes called "leisure batteries", are
something of a compromise between the two, able to be discharged to a greater
degree than automotive batteries, but less so than deep cycle batteries.
5.3) Fast and slow charge and discharge
When a battery is charged or discharged, this initially affects only
the reacting chemicals, which are at the interface between the electrodes and
the electrolyte. With time, these chemicals at the interface, which we will
call an "interface charge", spread by diffusion throughout the volume of the
active material. If a battery has been completely discharged (e.g. the car
lights were left on overnight) and next is given a fast charge for only a few
minutes, then during the short charging time it develops only a charge near
the interface. After a few hours this interface charge will spread to the
volume of the electrode and electrolyte, leading to an interface charge so
low that it may be insufficient to start the car. On the other hand, if the
battery is given a slow charge, which takes longer, then the battery will
become more fully charged, since then the interface charge has time to
redistribute to the volume of the electrodes and electrolyte, and yet is
replenished by the charger.
Similarly, if a battery is subject to a fast discharge (such as
starting a car, which is a draw of some 200 amps) for a few minutes, it will
appear to go dead. Most likely it has only lost its interface charge; after a
wait of a few minutes it should appear to be operative. On the other hand, if
a battery is subject to a slow discharge (such as leaving the car lights on,
which is a draw of only 6 amps), then when the battery appears to be dead it
likely has been completely discharged.
6) Valve regulated lead acid batteries
The Valve Regulated Lead Acid (VRLA) battery is one of many types of
lead-acid batteries. In a VRLA battery the hydrogen and oxygen produced in
the cells recombine back into water. In this way there is no leakage. Since
VRLA batteries don't require (and make impossible) regular checking of the
electrolyte level, they have been called Maintenance Free (MF) batteries.
However, assemblies of VRLA cells do require some maintenance.
VRLA types became popular on motorcycles because the acid electrolyte
is absorbed into the medium which separates the plates, so it can't spill.
This medium also lends support to the plates which helps them better to
withstand vibration.
The electrical characteristics of VRLA batteries differ somewhat from
wet-cell lead-acid batteries, and caution should be exercised in charging and
discharging them.
7) Exploding batteries
Excessive charging of a lead-acid battery will cause emission of
hydrogen and oxygen from each cell, as some of the water of the electrolyte
is broken down by electrolysis. Wet cells have open vents to release any gas
produced, and VRLA batteries rely on valves fitted to each cell. Wet cells
may be equipped with catalyic caps to recombine any emitted hydrogen. A VRLA
cell will normally recombine any hydrogen and oxygen produced into water
inside the cell, but malfunction or overheating, may cause gas to build up.
If this happens (e.g. by overcharging the cell) the valve is designed to vent
the gas and thereby normalize the pressure, resulting in a characteristic
acid smell around the battery. Valves can sometimes fail however, if dirt and
debris accumulate in the device, so pressure can build up inside the affected
cell.
If the accumulated hydrogen and oxygen within either a VRLA or wet
cell is ignited, a small explosion is produced. The force is sufficient to
burst the plastic casing or blow the top off the battery, and can injure
anyone in the vicinity and spray acid and casing shrapnel to the immediate
environment. It is surprising how powerful an explosion can be caused in the
small air space above the electrolyte. When one cell explodes, it sets off a
chain reaction in the rest.
VRLA batteries usually show swelling in the cell walls when the
internal pressure rises. The deformation of the walls 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 isolated and discarded,
taking great care using protective personal equipment (goggles, overalls,
gloves, etc.) during the handling.
7) Environmental concerns
Currently attempts are being made to develop alternatives to the
lead-acid battery (particularly for automotive use) because of concerns about
the environmental consequences of improper disposal of old batteries and of
lead smelting operations. Ni-Mn is already widely used in hybrid vehicles.
Newer technologies are unlikely to displace lead-acid batteries owing to the
much greater cost of potential alternatives. Nickel and Manganese are
considerably more expensive than lead or antimony. At current (May 2008)
prices quoted on the London Metal Exchange, lead is about ten times cheaper
than nickel for example.
Lead-acid battery recycling is one of the most successful recycling
programs in the world, with over 97% of all battery lead recycled between
1997 and 2001. 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.
8) Additives
Many vendors sell chemical additives (solid compounds as well as
liquid solutions) that supposedly reduce sulfate build up 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 should not be expected to
have normal life expectancy. Residual EDTA in the lead-acid cell forms
organic acids which will accelerate corrosion of the lead plates and internal
connectors.
Active material (the positive plate lead dioxide and negative plate
spongy lead) changes physical form during discharge, resulting in plate
growth, distortion of the active material, and shedding of active material.
Once the active material has left the plates, it can't be restored into
position by any chemical treatment. Similarly, internal physical problems
such as cracked plates, corroded connectors, or damaged separators can't be
restored chemically.
9) Maintenance precautions
One precaution in workshops that handle large lead-acid batteries is
a supply of ammonia solution to squirt on any spilled battery acid, to
neutralize it. Surplus ammonia, and water, evaporate off, leaving a deposit
of ammonium sulfate.
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