Resurrecting this thread. I have been flying the
M2P since release, and had a number of batteries swell, but until today, I had never seen a battery overheat and start melting the plastic. 68 charge cycles prior to this flight. Manufactured Dec, 2019, and put into service July, 2020. Same heat location as the OP experienced - left rear. It smells like a cooked capacitor or resistor (I wasn't going to open it to look), and got so hot that the battery compartment on the AC deformed slightly as well.
The battery powered up the AC upon insertion rather than requiring me to power it up. Based on that alone, I decided to fly an easy test close to me so that damage would be limited if the battery failed. After a 7min, 20sec flight with a max height of 58' and a max speed of 15.68mph, I landed, and immediately smelled something burning. The battery appeared to power off, but the AC did not. I pulled the battery. The back end was too hot to touch, and was slightly deformed by heat. The base of the battery (where it would show signs of swelling if that had happened) was barely warm.
I then tested the AC with two other batteries. The first flight was gentle and close to me. The battery remained fairly cool. The second flight was 23,879' in total, with a max speed of 45.23mph. The battery was at 46% when I landed, and was warm, but not hot.
I'm fairly confident that with no history of battery issues on any of the five
M2P AC I use, this battery is flawed.