Hi RR..
If one stops to consider the method of construction of the Batteries, then reviews the Failure Analysis Associates link, for spontaneous failures, there are some possibilities remaining to explain the failures.
The construction of each battery requires delicate manual handling, perfectly clean ingredients, and materials that perform within narrow limits, in the interest of performance.
Each very small "shutdown event" increases the internal resistance, and lessens the capacity of the battery. As capacity decreases, demand increases, (as a percentage of "available") exacerbating the failure path.
Ever suspicious, I return often to the "150" Field replaced Batteries. Something was wrong with them, so there was some cause that created the "problem".
Did Boeing replace three batteries per fleet unit without knowing the cause for failure? That would be most troubling.
But if they knew the cause, how is it NTSB does not know it? Even more troubling.....
And why would NTSB use the associated Battery (MAIN) from the incident aircraft as an exemplar?
Budget trouble? (
)
cheers