During APU start, the Battery Charger Relay opens to prevent the ground service bus from charging the battery during APU Start. The Battery Charger Relay protects the battery from overheat/melt down during heavy draw on the battery. As the battery is recharged after APU start, a thermal sensor in the battery tray will change the charge from a constant to a trickle to prevent battery overheat as well. The DC meter is monitored for a large draw to ensure the Battery Charger Relay opened when the aircraft AC busses are being powered.