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At 12.6 volts, a fully charged battery stores and supplies the electrical energy needed to crank over and start the engine. However, once the vehicle is running it is the alternator that takes over and continues to supply the energy that operates the vehicle. The battery is much like the reservoir in a city water supply system, there is a flow in-to and out-of the battery.

Mechanics' Corner: More Technical Detail

An automotive battery is an electro-chemical device that uses a mixture of sulfuric acid and water that surrounds lead plates in six separate areas called cells. Ideally, each cell develops 2.1 volts for a total of 12.6 volts. The physical size and electrical capacity of the battery are vehicle-specific.