Hi,
In first NTSB briefing Deborah Hersman emphasized WHAT and WHY on BOS 787 incident.
WHAT: A short circuit inside a cell.
Problem now (WHY) seems much more complex. Even impossible to conclude:
1) PCB´s probably were FUBAR
2) Cell voltage history (before and after thermal runaway) probably were lost
If this is true, the only way to recover cell voltage history would be if battery voltage was measured and recorded. (NOT THE DC bus as happens in a conventional plane) Remember that between the cells plus terminal (the right position to measure) and the bus you have a relay/contactor and a diode module (this one specific to 787, because using Li ion that only "goes to the bus" if it drop below, i estimate 30 V)
So, if the voltage at battery plus terminal of cell # 8 was not measured or recorded NTSB will have two possibilities:
1) A cell failure from a non electric reason. (process, etc.)
2) A cell failure from a cell voltage balancing failure when battery was being charged. (Series charging, IMO not the best approach)
Considering
JA829J started flying few weeks before the incident, very probably no other battery was used (and replaced) in that plane. So not possible (very probably) to look to other battery used in (APU) this plane.
So i don´t expect a conclusive WHY from NTSB for this particular battery failure. Simply i cannot imagine how to conclude. (the WHY)
The fact they are going to see process (manuf., design and chargers) is an indication they had no means to find the WHY in using the shortest procedure: Looking to the smoking gun.
On the question in the Title: What concerns me most is the FAA design review. A mere change to Ni Cd could not be the only required change.
