@Lyman
Did Boeing replace three batteries per fleet unit without knowing the cause for failure? That would be most troubling.
But if they knew the cause, how is it NTSB does not know it? Even more troubling….
In the absence of external ground power, the 787 Ship's Battery can provide power for functions including refueling the plane or operating the brakes during towing. The APU battery can provide power for functions like running / anti-collision lights during towing.
Almost all of the battery failures were caused by ground personnel running these functions off the batteries for longer than Boeing's guidelines called for. This caused the batteries to discharge to a state that tripped safety systems to prevent over-discharge and/or damage to the battery. Said batteries were removed from the plane, re-charged at the airline's maintenance facility or an appropriate MRO, and then returned to service.
@inetdog
According to the description in the report, the controls for the APU are powered only from the APU battery bus.
The battery bus might also be powered from the charger (BCU) if the battery goes open circuit, but apparently when the battery fails in a short circuit mode or high enough level fault alarms go off the battery bus cannot receive external power and so the APU cannot run.
This is evidently a Boeing design philosophy. I have been informed that the 777's APU will also shut down if the APU battery fails and on the 767, turning the APU battery switch to the off position while on the ground will shut down the APU.