Since multichannel engage occurs at 1500ft or below, with the above conditions, there will be no bus isolation request and no drained battery.
This certainly makes for a good debate. You might like to discuss this with your fellow pilots/engineers/tech experts.
Irrespective, as Spanner Turner says, during bus isolation, the Battery Charger is providing power to the Hot Battery Bus, Static Inverter, Center Bus, etc. The Battery Charger switches to TR mode when the battery relay closes (as part of bus isolation). So where does the Battery Charger get its power from.... The Battery Charger is provided with power from the Right AC Bus. So, in reality, two FCC's are being powered by the Right Bus.
I believe the only time the battery is really used is if there is bus failure below 200'. Thus satisfying the independent source rules.
Incidentally, I've been told by pilots from two different airlines, that it's not a good idea to push the APP switch when you are using the VHF
comms, as there will be a temporary loss of signal. This would also suggest that bus switching occurs whenever the APP switch is pushed, rather than at 1500'.
BTW, do the FCC's need valid ILS signals for all three CMD switches to light up... or just a selected frequency/course? (Haven't had the chance to try this in the sim).
Rgds.
NSEU