Hi LEM,
OK, anytime the battery is switched ON, without AC power applied to the airplane, both the main and auxiliary batteries will power the standby power system.
Consider, if the external ground aircon was connected to the airplane, the battery is required to be on to power the protective dc circuits; anytime the APU is started the battery needs to be on; if both battery and external power are on, and the external power has some sort if hiccup, as it occasionally does, standby power will come on.
When standby power comes on, it powers all of the Captns side navigation/approach/communication systems (and his clock), and is good for an hour at a stretch!
Selecting the Standby Power switch to OFF, in the secure checklist will alleviate all these possibilities, and both batteries will most probably be fully charged when you arrive to fly that night flight.
Our normal procedures also has the Captn selecting standby power to auto during his cockpit prep, noting the OFF light goes out.
Cheers, FD
