I too have seen this problem pop up in the last few months. I have 4 batteries for my
M2P. Have had them since Sept 2018. 2 have moderate swelling 1 serious swelling and the last with very little. The last time I flew, upon landing the battery was actually popping out on one side. Very bad. That battery is getting retired. The drone could have fell from the sky. The batteries seem to puff up more when they get hot. I may begin zip tying the batteries in just for safety until I can get more. Is it normal for these expensive batteries to swell after 18 months ??? I'm beginning to suspect the charger is charging too fast and damaging the batteries.