The NTSB hearing on the batteries is taking place right now...very, very interesting...
National Transportation Safety Board
(and damn, the NTSB chair is a very striking woman!)
in the morning, there was some very revealing data..
Foremost, in my mind, was that with the relative small number of large batteries produced (like on the 787) that there is not enough data to support the 10 minus 6 failure rate protection. The large batteries are produced in small batches, which there just isnt enough data yet to form a solid failure rate. The panel speaker from NASA noted a 3% initial rejection rate on the batteries they get, which is far above 1/10MM.
The battery electrolyte being flammable, not combustible, was a big issue for many on the NTSB Board
Another issue for the 787 battery was the size/shape, several panel speakers noted the large shape, with expansion/contraction from charge/discharge was an issue, and that the rectangular shape, in expansion/contraction, can cause issues. The panel speaker noted that this can cause failures, ie internal shorts, that are not picked up during the initial manufacturers testing of the battery prior to ship.