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Lead-acid battery

Lead-acid batteries are the most commonly used rechargeable batteries today. They also represent the oldest design with one of the worst energy per weight ratios. However, 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 2V nominal voltage. Each cell contains (in the charged state) electrodes of lead metal (Pb) and oxidized lead (PbO) in an electrolyte of about 37% w/w sulphuric acid (H2SO4). Modern designs have gelified electrolytes. In the discharged state one electrode turns into lead 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 13.8V (for six element car batteries). Their capacity will severely suffer from deep cycling. Especially designed deep-cycle cells are much less susceptible to this problem, and are required for applications where the batteries are regularly discharged.

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.