The BAT DISCHARGE light illuminates if the discharge is more than 5 amps for 95 seconds, 10 amps for 25 seconds or 100 amps for 1.2 seconds.
Both the main and the aux battery are monitored.
either the battery charger is to be suspected inop or its bus is not powered.
On ground, ELEC light will also illuminate with this and additional information will be available on the LAD!