The car's initial source of electricity is a battery. Naturally, its most important function is to start the engine. Once the engine is running, an alternator takes over to supply the car's electrical needs and to restore energy to the battery. A 12-volt storage battery consists of layers of positively and negatively charged lead plates that, together with their insulated separators, make up six two-volt cells.

The cells are filled with an electricity-conducting liquid (electrolyte) that is usually two-thirds distilled water and one-third sulfuric acid. Spaces between the immersed plates provide the most exposure to the electrolyte. The interaction of the plates and the electrolyte produces chemical energy that becomes electricity when a circuit is formed between the negative and positive battery terminals.

There are several types of automotive batteries. They range from maintenance free batteries to batteries which require maintenance. Batteries that require maintenance need to have distilled water added periodically. In most cases, a maintenance-free battery will be marked as such on top. Some batteries have a charge indicator (a small round window on the top of the battery). If the window is green or blue, the battery is fully charged and functioning properly.