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Lead-acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type of rechargeable battery. more...
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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, makes them ideal for use in cars, as they can provide the high current required by automobile starter motors. 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. They are also used to power the electric motors in diesel-electric (conventional) submarines.
Electrochemistry
Each cell contains (in the charged state) electrodes of lead metal (Pb) and lead (IV) oxide (PbO2) in an electrolyte of about 37% (5.99 Molar) w/w 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):  Cathode (reduction): 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.
The following are general voltage ranges for six-cell lead-acid batteries:
Open-circuit (quiescent) at full charge: 12.6 - 12.8 V;
Open-circuit at full discharge: 11.8 - 12.0 V;
Loaded at full discharge: 10.5 V.;
Continuous-preservation (float) charging: 13.8V for gelled electrolyte; 13.5V for AGM (Absorbed Glass Mat) and 13.4V for flooded;
Read more at Wikipedia.org
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