The battery relay light is for the following:
If you have a Nicad battery and it goes hot, the procedure is to turn off the battery switch and land as soon as practical. This isolates it from the generator. It only works if you have a Nicad battery.
When the battery switch is turned off and the relay light is illuminated then it means that the generator is still charging the battery because the relay has failed in the on position. Therefore the risk of thermal runaway and possible battery explosion is elevated. Land as soon as possible having turned off the generator and battery switch (yes, giving yourself an electrical failure). If above 6000 feet pressure altitude, if possible, get below it as turning off the battery and generator will remove power form the boost pumps. If you are not able to get below 6000 feet.........use your best judgement!