Car Battery

Most cars use a 12-volt lead-acid battery: flooded, EFB (enhanced flooded) or AGM. Stop-start cars need EFB or AGM types because they are cycled much harder. Electric and hybrid cars also have a 12-volt battery for their control electronics, separate from the high-voltage traction battery.

Battery life depends on heat, short trips and electrical load; many last around four to six years. A healthy battery at rest reads about 12.6 volts. Starting the engine needs a large current for a few seconds, which is why a weak battery often fails first on a cold morning.

Common types are flooded lead-acid batteries, EFB (enhanced flooded) batteries and AGM (absorbent glass mat) batteries. Cars with start-stop usually need EFB or AGM, and the replacement should match the original type and capacity.

Chemical reactions between lead plates and sulphuric acid store and release energy. When the engine runs, the alternator pushes current back into the battery to recharge it. Capacity is given in ampere-hours (Ah) and starting power as cold cranking amps (CCA). On many modern cars a battery sensor on the negative terminal tells the control unit how charged and healthy the battery is.

A weak battery causes slow cranking, clicking, warning messages and odd electronic faults, because many control units react to low voltage. On many modern cars a new battery must be registered so the charging system adapts to it. Batteries usually last several years, but short trips, cold weather and long periods of standing shorten their life. On some cars a new battery must be registered with a diagnostic tool so the charging system treats it correctly.