Have Boeing just been too agressive in their power & weight demands of the L-ion batteries? Most battery fires appear to be caused by overly thin insulation (reduced weight) and squeezing the last milli-watt of charge (the last few percent) into the battery - necessay for mobile phones & laptops, but for planes?
Would a few grams more weight and a few hunderd less mill-watt hours per battery really make a difference on a plane?
How long do Boing expect their craft to operate in-flight without main generators, or on the ground without an APU?