RR,
And for anything in ground too. So the question is: Why the cell(s) started to overheat?
Agreed. Reading through this thread there are many cogent theories as to why the overheat began and I have to say most of them seem at least possible.
The main question for me is: why did batteries whose technology was known to be vulnerable to catastrophic thermal runaway (more than virtually any other kind of cell) get put into a configuration that sent the whole lot up in flames if there was a *single* failure? Who in their right minds would install that in an aircraft that could be 3hrs from the nearest airfield?
If it had been designed to contain an individual cell going AWOL, then you'd get something like a status message "ELEC BAT MAIN SYS" which would show reduced redundancy in flight and maybe no dispatch at the other end until the problem had been sorted. You don't expect (or want) a large percentage of the chemical energy in the whole battery being released over a short period of time inside the aircraft...
Looking at it from a statistical POV, if all it takes is a microscopic rupture in a nanometre-scale film in one cell to trigger a destructive event, when there are 800 787s flying 18hrs a day carrying (insert number) of cells, how likely does this become?