How is done the filtering of the busses? There are capacitors banks
along the busses? Or any "sudden drop" of voltage makes the battery
supply the energy (through the hi speed switch - the diode). If there
are spikes (sudden negative going voltage levels) in the busses the
battery is not as "offline" as previously thought.
I think your'e chasing a red herring here, or something, as there
have been similar failures in functionally different areas: Standby
power and APU start.
This would suggest that the problem is with the battery / charger
subsystem, as it's the only thing in common,
afaik...
Regards,
Chris