@fgrieu,
So if it is not stored in NVM, nor recorded in the FDR, where does the data about cell overdischarge goes?
Presumably notable events are input to the aircraft's on board diagnostics system and maintenance logger (there's probably correct words for that kind of thing that I don't know).
As an electronics engineer I find it astonishing that the battery management systems don't retain a running log of the battery's operating conditions and status. It's trivial to do that sort of thing, and that sort of data is invaluable when you have problems like this. You'd have to be spectacularly unimaginative or tight fisted not to design that in, especially as it would cost $1.50 in parts to do it.
@ITman
So bench testing is superior to flight testing and associated ground handling..?
It can be, but it depends on knowing what the full range of possible operating conditions are, including severe extremes caused by other problems (eg loss of bay heating). I suspect that Boeing and partners have a rough idea on what those are, but it smells like they've not got many measurements or logs of what they actually go through. What's the bet Boeing lost some ancient battery specialist from the payroll and didn't replace him/her?
That in turn suggests that the spec for the battery was drawn up incorrectly, which is going to be down to a lack of experience on someone's part. That is an assumption backed up by everyone being mystified as to what's going on. The batteries are passing their existing bench test, but that's clearly not thorough enough. Anyway Boeing seem to want to fly the thing just once. That's hardly likely to fill in any knowledge gaps; only sustained operational use with full logging of everything will reveal what's going on.
Thing is, they've changed things. And worse still commercial pilots are going to be very wary of the battery, so they're going to make as little use of it as possible. It might be that the causal conditions don't arise again, meaning that Boeing may never learn what the problem actually is. And that will always leave a nagging doubt about the whole aircraft.