I had a battery in my P2 running the battery down. It was just on with motors not running. I smelled something burning so I stopped it. However, what complicates the situation is that I was also rendering some video on my laptop and the laptop fans were really going strong, so maybe I smelled that? I don't know. I took the battery out of the P2 and it may be puffy. It's possible that just the wrapper is slightly inflated but, that said, it is definitely not as flat as my three other batteries. Does anyone have a good, clear pic of a puffy P2 battery?