@RR_NDB
First i need to clarify some points to prepare my comment.
1) Destruction of # 3 was by what? Internally generated heat (due subcell inbalance, dendrites, voltage), ohmic losses due thousand Amps flowing to case (and ultimately to ground), or what?
I continue to rule out thermal runaway as cause of Primary failure. My scenario is same as at BOS, the same pattern. Primary failure was in #3 (#5 at BOS) due to deterioration of unknown origin. Secondary failure overcharge and thermal runaway of several other cells. Tertiary failure is total destruction of #3, how the pole can burn away, leaving the nut unscathed, beats me. The cause of "deterioration" leading to Primary failure remains as elusive as ever. By now, NTSB could have said something, perhaps them looking for micro-shorts by testing last week gives a hint toward the short of a cell as the cause of Primary failure.
2) On ground THICK wire you has two details: One seeming mechanical and another VERY PROBABLY caused by excessive current. Please refer to the equivalent circuit where i put the (highly probable due evidences) short circuit path.
Yes, it could be that the frayed, not severed wire burned up the way you suggest. It could also be that the severed wire caused shorts on a BMU PCB. Now, that would be really awkward and destroy the pattern of both batteries and feed sabotage conspiracies.
3) Severe damage shows a PATTERN. Itīs up to us to establish a model rhat if is robust enough could explain most. I am just taken into account everything. Could we discard sabotage? Answer is NO. I prefer look to all details and balance them in the best proportions.
I think it is tertiary damage pattern, like structural breakup, it can go any way.
4) Shorting cell? Which one? Due over voltage, improper charging? Temperature of individual cell(s) not being measured?
#3, (#5 at BOS), deterioration of cells by mishandling after assembly** / by maintenance or mismanagement by BMS during flight operations.
** I don't know how battery could be safely assembled when cells carry a charge. Any mistake like a short would lead to a firework or damage. Perhaps, battery is assembled while cells not carry a charge, and are charged after assembly.
I am not confident about cause of deterioration leading to Primary failure.
Mishandling like for example resetting BMU cutout of deep discharged battery, thereby charging damaged cells would be a convenient and fixable explanation. Mismanagement by BMU would be harder to fix.
If I would want to figure evtl mismanagement out, I would connect transient processing by way of a 8 channel differential 12bit transient recorder at 1megasamples/sec to the battery, one channel per cell and perform real-time transient analysis to find unique voltage transients on cells.
I feel confident toward the Secondary failure pattern, it is a very tough job for BMS to detect cell short in a timely manner. That will have to be fixed. It may well take something like the above mentioned transient processing to be responsive enough. Another way could be an infrared camera in the case. BTW, Temperature sensors would be not much use as the response is way too slow.
Tertiary failures - breakup - I don't really think about.
It all may well boil down to lack of understanding in a highly decentralized operation. Examples are that nothing seems to have been done about deep-discharged and failed batteries over many months.
Edit: BMU -> PCBs in battery box. BMS -> BMU + charger + all other related functionality.
@FlightPathOBN
I would suspect the way the cells are wired as well...not with bus bars, or ground straps, but simple braided wire...
and I certainly would not consider this 'thick' wiring by any definition...
The cells are connected using at least 350A rated bus bar and the wires are only for monitoring and balancing at currents below 5A.
Edit: I also would like to see CT scans of in-service as well as of failed batteries.