Latetonite,
In normal circumstances the charger doubles as a fourth TR and supplies DC to the HBB and the SHBB and that is why you don't normally see a discharge on the battery. If the charger fails to do that then the battery will be supplying the power for those two, hence the discharge.
That is my understanding of the schematic I have in the FCOM. If I have misinterpreted it I look forward to being corrected.
Cheers.