sudomod

I am fitting a Odroid Xu4 into a Wii U and was planning on using the RetroPSU for power and using the onboard button for safe shutdown. I had soldered a battery to the pads and left the project sitting for a couple days, only to come back to a swollen battery. I extensively checked for shorts to find none, and there was no external components apart from the battery attached to the board. What caused the battery to swell and should I use another circuit for safe shutdown? I'm planning on using The Retro Arena's OS for my project if that changes anything.