A car battery powers the car's engine, lighting system and all electrical & electronic accessories. The battery is usually charged by an alternator, which converts the mechanical energy from the engine into electrical energy. The battery will get old along the years and will not be able to hold a charge. When selecting the replacement battery, you need to take note of the size and reserve capacity. Right size is required so that the battery can fit securely in the battery compartment and is well connected to the terminals.