kiskaloo:
Or must the APU battery be energized and available in order to start the APU on the 787?
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.
Lyman:
And why would NTSB use the associated Battery (MAIN) from the incident aircraft as an exemplar?
Similar use history and manufacturing batch, maybe?
Easier than searching other aircraft in use to find a qualified exemplar. Or is the manufacturer required to archive one or more sample batteries per batch?
The exemplar has (so far) only been subjected to non-invasive examination.