The wet cell lead acid battery is the main storage device in automotive use. An automotive battery can supply very high discharge currents while maintaining a high voltage - useful for cold starting. It gives a high power output for its compact size, and it is recharge-able.
Lead-acid batteries, invented in 1859 by French physicist Gaston Planté, are the most commonly used rechargeable batteries today. They also represent the oldest design with one of the worst energy-to-weight ratios, although the power-to-weight ratio can be quite good. Also, the energy-to-volume ratio is good compared to other types of batteries. They are cheap and can supply high surge currents needed in starter motors. Every reasonably modern car uses a lead-acid battery for this purpose.
Lead-acid car batteries consist of six cells of 2 V nominal voltage. Each cell contains (in the charged state) electrodes of lead metal (Pb) and lead(IV) peroxide (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. Specially designed deep-cycle cells are much less susceptible to this problem, and are required for applications where the batteries are regularly discharged.
The energy to weight ratio, or specific energy, is in the range of 108 kJ/kg (30 Wh/kg).
The chemical reactions are (charged to discharged):
Anode (oxidation): 
Cathode (reduction): 
Because of the open cells with liquid electrolyte in most cheap car 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 sulphuric acid.
Source: CDX Global & Wikipedia - en.wikipedia.org
