737NG APU Start Issue
Just before arriving on stand we attempted to start the APU. Both engine driven generators were online.
The APU was slow to start and the EGT climbed to and stayed at around 900-1000C. We then had a MASTER CAUTION ELEC along with the BAT DISCHARGE light.
The APU uses power from transfer bus 1 to start when AC is available. Why in this case did we get the BAT DISCHARGE light when both generators were online?