The alternator is designed to keep the car battery fully charged, it is not designed to charge flat batteries. They are on the whole very reliable and will spin away for years without failing. They work by producing alternating current (hence alternator). The current is produced through a series of windings, which sit in a steel (normally) housing around the one moving part of an alternator. This winding is called the stator, and the part that spins – the rotor.