Lyman:
The NTSB isolated the #6 cell as the source of the initial runaway. Working
backward, where do you see the failure as initiating?
Imho, the precursor to the initial runaway should have been detected by the bms
as an excessive temp rise or temperature gradient in that cell. If each cell's
temperature is being monitored, detection of an overtemperature condition
should have been part of the process, just as temperature monitoring is critical
to limit charge current to a safe value.
I know there's been some discussion about whether a sensor on each cell would
be fast enough to detect the rise, but the cells do have thermal mass and
unless a short circuit or very high charge current were applied, the temp
rise over time would be such that a sensor should detect it. As the cells have
electrolyte, the thermal resistance from cell core to case should be fairly low.
Temp rise timescales might be in the low minutes under normal operating
conditions. Plenty long enough to be detected by a sensor on the cell casing,
but maybe not so if that sensor were several inches / 10's cm away on the
enclosure wall...