My point being that if the Main and APU batteries are in casual use in flight, and the FAA has specified a "minimum Charge" to adhere to the definition of "BACKUP", then one or the other or both are constantly being charged, discharged, and for most of the time, the Battery is merely a "conductor" in line, neither powering or being reharged alone, but in constant connection.
My assumption is that the "Minimum charge" is somewhere near "Maximum Charge", so the Battery will be in line enroute. Especialy so with the MAINBAT.
Once the APU starts, can it shutdown whilst the APUBATT is below minimum charge? Since the APUBATT is charged by the vdcSG, the BATT must be restored to full charge, or may not be able to start the APU (a second time) in an emergency.
Having started the APU, perhaps draining the Battery, how can the Battery comply in flight?
This is the thing. Using a Backup Battery in its emergency configuration, not in an emergency, fails even the definition of BACKUP.......