Lead-acid battery

Lead-acid batteries, invented in 1859 by French physicist Gaston Planté, are a type of galvanic cell and are the most commonly used rechargeable batteries today. They also represent the oldest design with one of the lowest energy-to-weight ratios besides the Nickel-iron battery, commonly around 30 Wh/kg. Also, the energy-to-volume ratio is low compared to other types of batteries. The power-to-weight ratio can be quite high, however. They are relatively low-cost and can supply high surge currents needed in starter motors. Every reasonably modern car uses a lead-acid battery for this purpose. 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.

Lead-acid car batteries for a '12 volt' system consist of six cells of 2.1 V nominal voltage. Each cell contains (in the charged state) electrodes of lead metal (Pb) and lead (IV) oxide (PbO₂) in an electrolyte of about 37 % (or 6-12M) w/w sulfuric acid (H₂SO₄). Some modern designs have gelified electrolytes. In the discharged state both electrodes turn into lead(II) sulfate and the electrolyte turns into water. (This is why discharged lead-acid batteries with gelled electrolytes can freeze.)

Lead acid batteries designed for starting service, such as those in used in most automobiles, are not designed for deep discharge. Deep cycling will result in capacity loss and ultimately in premature failure, as the electrodes disintegrate due to mechanical stresses that arise from cycling. A common misperception 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.

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. Marine 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.

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.

The following are general voltage ranges for six-cell lead-acid batteries:

Quiescent(open-circuit) voltage at full charge: 12.6 V
Unloading-end: 11.8 V
Charge with 13.2-14.4 V
Gassing voltage: 14.4 V
Continuous-preservation charge with max. 13.2 V
After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V.

The chemical reactions are (charged to discharged):

Anode (oxidation): $\mbox{Pb} (s) +\mbox{SO}_{4}^{2-} (aq) \leftrightarrow \mbox{PbSO}_{4} (s) +2e^- \quad\epsilon^o = 0.356 V$

Cathode (reduction): $\mbox{PbO}_{2} (s) +\mbox{SO}_{4}^{2-} (aq) +4\mbox{H}^++2e^- \leftrightarrow \mbox{PbSO}_{4} (s) +2\mbox{H}_2\mbox{O} (l) \quad\epsilon^o = 1.685 V$

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 extremely explosive mix. This should be avoided. Caution must also be observed because of the extremely corrosive nature of sulfuric acid.

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. 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. Effective Lead pollution control system is a necessity for sustainable environment. There is a continuous improvement in battery recycling plants and furnace designs for greater efficiencies. These recycling plants are ecology friendly as they follow all emission standards for lead smelters, but new methods should be devised or alternatives developed to the lead-acid battery so that lead pollution can be reduced to an essentially negligible amount.

Lead-acid batteries react less violently to fire exposure than Nickel-cadmium batteries thus they are used in emergency lighting in case of power failure.

Other 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 cells are used in back-up power supplies for alarm and smaller computer systems (particularly in uninterruptable power supplies) and for electric scooters, electrified bicycles and marine applications. Unlike wet cells, gel cells are sealed, so they are less prone to spilling and do not require maintenance of electrolyte levels.

Absorbed glass mat (AGM) cells are also sealed and used in battery electric vehicles.

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-rechargable "B" battery for the plate (anode) voltage. A few sets also used a third (3 V–9 V with several taps) "dry" non-rechargable "C" battery for grid bias.

References

U.S. Department of Energy, Primer On Lead-Acid Storage Batteries ^* (pdf).
Environment Friendly Battery Recycling

External links

Rechargeable batteries | Automotive technologies

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Environmental Concerns

Other applications

See also

References

External links