In 787 the MAIN battery are kept charged separated from the bus by a diode: A "high speed switch" that is controlled by the bus voltage. When the bus drops below (estimated) 30 V the battery (if it is ON) supply current to the bus to maintain itīs voltage at required values. A low voltage in the DC bus would make the loads (electronics) fail.
Thats not how I read it.
The BDM (Bat Diode Module) is there to stop the battery being 'charged' from the Hot Bat Bus in the event the Hot Bat Bus is connected to one of the [live] main DC Busses. It's just a non return valve as a back up to a bus breaker/contactor.
The Battery Charger is supplied from a different source (Capts Instrument Bus).
In regards to the batteries, in the fwd EE bay, it appears this is run like a UPS, where the power is constantly being drawn through the battery system, to provide a clean source for the avionics, else there would not have been an issue in-flight. IF that is the case, the batteries are in constant use.
In your model what is the function of the diode module?
IMO the MAIN and APU batteries are kept OFF LINE.
Exceptions (batt.):
MAIN: DC bus below ~30 V (including obviously, negative going spikes)
APU: APU start or towing lights consumption (with no APU)
(Most of use after you enter the plane and starts to use electricity, if no GPU and before APU)
More or less correct.
The APU Bat will ONLY be used to start the APU if no other AC source is available.
The Main Bat again will only be supplying if main DC is not supplying.
The exceptions are: Towing Switch.
APU Bat supplies the Nav lights.
Main Bat supplies brakes, No.1 VHF Tx, etc.
On Bat refuelling.
Main Bat supplies fuel quantity indicating system and limited refuel valve operation.
For what it's worth my opinion is that the majority of battery replacements have been due to the towing and perhaps refuelling switches being left 'on' for too long.
