@grumpyoldgeek
Here's what I don't understand. I have read that the 787 batteries had been failing non-catastrophically at some enormous rate prior to the catastrophic failures. I have not seen a single word about the failure analysis of these batteries. What's up with that? Surely A) Boeing must have been concerned that there was a deeper problem as soon as 4-5 batteries had to be swapped out and B) There must have been some clue as to what was going wrong and that the possibility of a catastrophic failure was probably much greater than originally predicted.
In the majority of cases, the cause was ground crew operating the aircraft solely off battery power for periods longer than Boeing specified. This drained the batteries to a level where the safety systems in the BMU and BCS disconnected the battery to prevent continued discharging.
Once the battery was drained to this state, it could no longer be charged on the airplane using the airplane's charging systems. Instead, the battery had to be returned to Yuasa for recharging and refurbishment. As to why this was the case, I have not seen a definitive explanation. Perhaps because the airplane's battery system could not fully recharge the battery quickly enough. Perhaps Yuasa felt such a deep discharge required the battery cells be examined and replaced, if necessary.
One of the changes Boeing is making is raising that discharge cutoff limit. I would expect the limit will be raised to a point where the onboard charging system can fully recharge it in whatever time period is deemed necessary and therefore the battery will no longer need to be removed.