Lead-acid battery

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Image:Lead acid cell.jpg Template:TOCright Lead-acid batteries, invented in 1859 by French physicist Gaston Planté in favor of the European automobile prototype, 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, although the power-to-weight ratio can be quite high. Also, the energy-to-volume ratio is high compared to other types of batteries. 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 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 % w/w sulfuric acid (H₂SO₄). 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 can freeze.)

Lead acid batteries for automotive use are not designed for deep discharge and should always be kept at maximum charge, using constant voltage at 13.8 V (for six element car batteries). Their capacity will severely suffer from deep cycling, due to sulfation, or hardening of the lead sulfate. Specially designed deep-cycle cells are much less susceptible to this problem, and are required for applications where the batteries are regularly discharged. Also, 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.

A chemical compound in the form of tablets can be added to each cell to reduce sulfate build up, and improve battery condition; however the effectiveness of such treatments is subject to debate.

The following are common 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 energy to weight ratio, or specific energy, of conventional batteries is in the range of 108 kJ/kg (30 Wh/kg).

The chemical reactions are (charged to discharged):

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

Cathode (reduction): <math>\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</math>

Because of the open cells with liquid electrolyte in most lead-acid batteries, overcharging with excessive charging voltages will generate oxygen and hydrogen gas, forming an extremely explosive mix. This should be avoided. Caution must also be observed because of the extremely corrosive nature of sulfuric acid.

1 Environmental Concerns
2 Other applications
3 See also
4 References
5 External links

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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.

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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 6V 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 (45V–120V) "dry" non-rechargable "B" battery for the plate (anode) voltage. A few sets also used a third (3V–9V with several taps) "dry" non-rechargable "C" battery for grid bias.

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