RetiredF4
I think you are on the right track in noting Boeing's remarks on moisture.
From memory, I think the statement was phrased something like, “During testing, we have learned that it is possible for moisture in the form of condensation to form a very high impedance short-circuit between the battery and the case which can lead to stress and venting of cell(s)” Not the exact words but near enough from memory.
Boeing also said that the vaporized cell contents escaping from the box “only caught fire because it came into contact with arcing wires outside the box”. No explanation as to why they were arcing, but I assume it was the ground wire which was found burnt out.
Both these events are very similar to an idea in the link below and earlier links from the same contributor:
http://www.pprune.org/7691355-post785.html
From first-hand experience I know that water will accumulate in any
nearly closed container which is subjected to humidity and temperature cycles.
http://www.pprune.org/7746642-post1014.html
The Boston incident aircraft was operating between Narita, which has a daily high of about 50ºF and no dry season, and Boston, which is generally freezing in January. This particular battery had been in airline service for only 18 days and 22 flight cycles before it failed.
So I think that Boeing thinks that moisture was or could have been the likely cause of failure and that if they can keep moisture out that there will be no more early battery failures. The new enclosure is necessary for this purpose as well as to keep oxygen out and debris in.
The proposed enclosure and burst disk certainly has the potential of keeping out moisture if it is installed and maintained properly, e.g. no undetected leaks in the system.