Although we refer to a car's primary electrical storage as a '12V battery', its actual voltage figure isn't always 12 volts. In general, the exact voltage can vary anywhere from 12.6 volts to upwards of 14.7 volts. The former figure, 12.6 volts, is what's known as the resting voltage of the battery. With the engine turned off and when your car isn't running, this is the idle, default voltage of the battery. When your engine is running or while you're driving, this voltage could rise to between 13.7 to 14.7 volts. This higher voltage count is due to the alternator having to constantly charge the battery, and the 12V battery undergoes constant discharge and recharge. If the reading falls below 12.6 volts, then it's a sign that your battery is losing charge or it's dying.