I have a 2S 20C Losi 5000 mAh hard case battery that has started to puff up. It has always been fully charged before use and never run down more than 2000mAh. The cells are still and have always been within .02 volts of each other when balancing. This battery is about 9 months old and has never been abused, only used for 5 minute heat races. It has always been charged with a Triton Jr. at 5 amps. I noticed the swelling when I went to put it back in the chassis after recharging. Any ideas on what may have caused the swelling?

It could have impact damage.

Hope you are not using it anymore!!!!!

Nope, just need to safe it and dispose of it. Right now it is in a metal ammo box.

Discharge it down slowly with a nimh discharger down to a volt or so. Then put it in a bucket of salt water for two weeks and then throw in the trash.