The APU on the 777 operates the same way - if the APU battery becomes inoperative, the APU shuts down. ....
Kiskaloo
On B777 the APU battery can be removed with APU running as the controller receives power from the Main battery bus as well.
Strangely on B787 the APU controller has only the APU battery supply. Without that the APU shuts down.
I have no idea why this change. May be they wanted save the weight of a wire??