A normally open, magnetically closed switch in the output circuit of a DC generator, between the generator and the battery. When the generator voltage is lower than that of the battery, a spring holds the reverse-current relay contacts open, and the generator is not connected to the battery.

As soon as the generator voltage rises above that of the battery, an electromagnet in the relay closes the contacts and connects the generator to the battery. The more current flowing from the generator to the battery, the tighter the contacts are pulled together.

When the generator voltage drops below that of the battery, reverse current flows from the battery to the generator. When reverse current flows, the magnetic field holding the contacts closed is cancelled, and a spring pulls the relay contacts open. This disconnects the generator from the battery. If a DC generator control circuit did not incorporate a reverse-current relay, the battery would discharge through the generator when the generator voltage was lower than that of the battery.