Framer,
I gave a random figure of -7amps, does anyone know how long the battery would last with a low discharge like that?
Looking at the MRG it seems that there isn't much on the hot buses to consume power. I suspect that the battery drain rate would be merely slight compared to also having to power the battery, dc and ac standby buses.
LASJayhawk,
IIRC (and it's been 10 years since NG avionics school) The ADIRU's or at least #1 are on the hot battery buss as well...
In this scenario, the IRS's should still be powered by the AC transfer buses. If and when the battery does run out of juice the IRS DC FAIL lights will show, but both IRS' will still function normally on AC.