The end of the matter…well almost
Boeing and the FAA have agreed a fix for the Dreamliner’s lithium battery fire problem, which clears the way for the FAA to issue an Airworthiness Directive, and the Dreamliner fleet to get airborne. The fix includes a new stainless steel outer battery enclosure (and vent pipe overboard) as well as modifying the battery box to include vent ports that will eliminate the build up of hot gases and flame within the battery box, should a cell thermally run away. In the worst case a battery fire would be totally contained and all gases discharge outside the aircraft.
Boeing noted that with this design change battery thermal over temperatures events were ‘slow and measured’ versus their ‘quick and energetic nature’ in the unmodified battery units; you have to love the dry understatement of the Boeing engineers.
My presumption is that this amelioration of the thermal over temperatures was primarily due to the batteries being allowed to vent into the enclosure rather than into the interior of the battery box and onto one and other. As I’d previously suggested this was one of the coupling mechanisms that turned an individual cell thermal runaway into a potential battery level fire (I’ll stop being smug now).
What’s interesting about Boeing’s solution is that it’s addressing the consequences of a battery fire as much as the causes, which is quite unusual for the aviation industry, so I’d guess that the engineers haven’t been able to convince themselves (or the regulator) that they fully understood and had controlled the causal mechanisms, within the deadline given. Hence the stainless steel bathtub solution.
An interesting case study in the real consequences of experimenters regress and how unexpected emergent behaviour can arise from the tight coupling of system components.