Battery issues - new battery

I have my Gameshell from the kickstarter campaign. I can’t say that I use it regularly, battery died 2 years ago and from other topics on the forum it is recommended to use a Nokia battery. I bought one, but after a few weeks battery is swollen.
The thing that caught my attention was this. Since the day I first bought the device, its battery was running low even when the device was turned off. So the battery was draining wherever it was off.
What do you think is the best battery solution?