Huge amount of overthinking on this thread.
If you are down to one generator start the APU. That’s what’s its there for. If it fails to start the remaining generator will replace any loss from the battery.
What are you going to do ? Wait for the remaining generator to fail before you start the APU. I don’t think so.
Then start the APU when you KNOW you are discharging the battery ? That would be dumb.
Last edited by Alt Flieger; 8th December 2021 at 02:21.