Alternator The alternator is the engine's way of generating electrical power from rotational motion. The alternator is usually belt driven and resembles an electric motor. As it spins it develops alternating current electricity which is then converted to the direct current that the electrical system needs to function. Most alternators output around 14.4 volts when the engine is running. Some older and military systems operate on either 6 or 24 volts. The electrical current is used to drive all electrical accessories and charge the battery for starting. Alternators are rated by the maximum amperage that they can produce. Most new cars have alternators rated between 60 and 120 amps.