I went out of the country and left 7 lipos, 2S and 3S stored very precisely at 50%. I'd planned on being gone for 4 to 6 weeks. Was gone for almost 11weeks. All of them puffed a cell and it was the cell that was positioned up as they were sitting, ie the top cell. This was in spring, wife was still home, we keep temp at 77-79F with a humidity at a comfortable level, 45-55%. They'd never been run hot. The puffed cells were over 3v, so I was at a loss to understand why it happened.

I never encountered a rational explanation for it either. These were gen 1 lipos and built with some kind of hard, white encapsulation on the end with the tabs, I could'nt even salvage the remaining good cells. I had well over 100cycles on them so they def were not new. I use 80% for storage and weekly ck all my lipos with a dvm.