A car battery typically has a voltage rating of 12 volts. When the engine is off, the fully charged car battery voltage will measure 12.6 volts, which is known as "resting voltage". When the engine is running, the battery voltage will typically rise to 13.5 to 14.5 volts, as the battery is boosted to these higher levels by the alternator. The actual voltage supply can vary slightly depending on the batterys composition, age, testing circumstances, and charge. A healthy car battery voltage should be 10 volts or higher during starting, and a fully charged car battery voltage should be 12.6 volts or above with the engine off.