pgarner528
and then carry on flying for the rated ETOPS time.
The problem is; these battery failures (fire or not) don't directly affect ETOPS range. The batteries are either there or not when they are needed. Their failure probability per flight may depend on the duration of that flight. Or the failure mechanism could turn out to be cycle rather than time dependent. One 'bad' charge or discharge event could initiate the fault.
That we don't understand the mechanism in play means that the probability math needed to ensure extremely unlikely events will be difficult. And involve quite a bit of Boeing/FAA hand-waving.