Hi,
TURIN:
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.
Thats how I read it! Since first moment learned on use of BDM.
The Battery Charger is supplied from a different source (Capts Instrument Bus).
Yes. Designers put the important charger switch in LH side. APU switch was left to RH.
It's just a non return valve as a back up to a bus breaker/contactor.
The BDM assures the only path to charge MAIN batt. is from itīs charger. Why? Because Li ion batteries are far more critical WRT to charging. When i first learned on itīs adoption for 747 i was concerned. The BDM "solved the concerning". You have a good approach.
Return valve is a good analogy.
The APU Bat will ONLY be used to start the APU if no other AC source is available.
Sure! APU batt. Is the secondary (redundant) resource.
The Main Bat again will only be supplying if main DC is not supplying.
I.e. when there is no DC (lower than a given value) in the BUS. I.e. when nothing (from AC source) is feeding DC bus. In this condition MAIN battery supplies (automatically) required energy to the bus through the BDM. (if batt. switch is ON)
The exceptions are: Towing Switch. APU Bat supplies the Nav lights.
Main Bat supplies brakes, No.1 VHF Tx, etc.
If "DC bus requires it" BDM allows flow from MAIN batt. to the bus.
On Bat refuelling. Main Bat supplies fuel quantity indicating system and limited refuel valve operation.
Again, if DC BUS requires it from main batt. As i understand if DC BUS is receiving DC from other sources (from AC of GPU, APU or gennies) MAIN batt. remains OFF LINE.
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.
Towing as i imagine is light consumption and "short" duration. It would be more probable more replacement of MAIN batteries. IMO much more probable to be inadvertently discharged below design limits. (designed to preserve and to avoid recharging a yet compromised battery)
In laptop batteries the voltage span from "empty to full" is about JUST ONE VOLT in a 12 V pack (3 cells in series). Therefore a VERY conservative use.
Fact is, these cells are wonderful but requires a lot of care. From design to end of life. And can be dangerous throughout itīs (entire) life. Someone here in pprune commented on fires of Li batteries dumped in trash in UK.