someone queried whether the main battery could/should be used to start the APU on the 787. Isn't that one of its main functions ?
On this machine, I believe this is not the case. The main battery looks after the mainframe computers (Common Core System), while the APU battery is used to start the APU. My (vague) recollection of the 757 system is that the batteries are paralleled for standby-power functions, but I can't recall what they do when starting the APU on the gound.