One potential failure mode occurred to me this morning as I was enjoying the blizzard. Perhaps the battery failures are related to pressure cycling. Since the batteries are normally sealed, each flight cycle will put a pressure differential on the cases of the individual cells. This, combined with vibration and G-forces, could result in very small, but repeated changes in the cell plate spacing/position - especially with a rectangular plate design. This could then lead to internal shorting, especially when coupled with charging errors and/or high current discharge (e.g., starting the APU). I'm sure Boeing validated the battery system for pressure changes, but perhaps it's all about the number of pressure cycles.