I noticed swelling either before or after my third flight. That third battery already had a slightly weak latch. Found my 2nd battery had swelling too, but not appearing quite as bad. My first battery barely noticeable. I only had the three at the time. I did low altitude/low distance tests with the #2, #3 without issues, but a month or so after found deals on new replacements, so now #1&2 are used for ground work (firmware updates, checking app settings, etc) and #3 was taken apart. If I were to fly with #2 for flying, I would velcro it down. Probably would do same for #1.