The alternator converts mechanical energy from the spinning engine into electrical energy. It is the primary source of power for the vehicle while the engine is running, supplying electricity to the lights, radio, ECU, and ignition coils, all while recharging the battery. Inside, it consists of a rotor and stator within an aluminum housing. Symptoms of a bad alternator include dim headlights, a battery warning light, or a dead battery that cannot hold a charge.