[ROVERNET - UK] relay

[JOHN HALL]

Yes. If the car doesn't have meters, the idiot light is the only way to let
you know if the battery is charging or not. The relay is measuring the
current going to the battery from the alternator. It compares the polarity
difference between the battery and alternator. If the alternator output is
higher than the battery, the relay is held on and the light is off. If the
battery voltage is higher then the relay relaxes and the light is on. That
is why the light goes on when the ignition key is on but the engine is not
running as when starting up. If the light doesn't come on when starting it
is usually a faulty relay.