Are you charging the batteries inside with cool air and then storing them outside in the heat for any substantial amount of time?


Heat will cause voltage to spike and that can damage the cells.


TheJANG from URC shared a story where he charged a handful of brand new LiPos inside in house, then packed his gear into the trunk of his car and drove 2 hours to the beach only to discover every battery was swollen with cracks on the seams of each battery where they were never used a single time!


Generally, you want to charge them at the ambient temp where they will be used if you want to minimize risk of damaging the cells.