An alternator is a machine that makes electricity that keeps changing direction — what engineers call alternating current, or AC. The one most people meet is in a car: while the engine runs, the alternator makes power for the lights, the radio, and for putting energy back into the battery. Though actually, everyday talk often just says generator for this, and in a garage that is usually what people mean — but in a careful science sense a generator can be a broader kind of machine, and the car part built for this job is an alternator. When the car is off, the battery does the work; the alternator earns its keep while you drive.