Robin Hoods Playground

I had a battery modded to be a pandora battery and its been sitting on my shelf for a few months cause I have not used it. Went to use it yesterday and its all swollen up! The sides of it are all puffed out and its about 15% fatter than it used to be.

Does anyone know what caused this???

no but it sounds like it has date with the trash, don't try and use it

Already tried it, doesnt switch on.
It wouldn't fit in the battery slot anymore

Throw it away, it may have been left in direct sunlight or something, but you should get rid of it, it can explode, or start leaking, and in both cases that would be a bad thing.