My new battery become swollen within 1 week of usage

last month the B button of my N3DS doesn't register. After looking up quick tutorial on youtube I managed to fix it. The problem is when I try to fix the button, I tried to test if my fix work so I put on the battery, held it with just my hand (without putting all the case) and turn the DS on, it was a little bit hard and my hand kinda slipped I saw a little spark coming from the terminal that connects the battery. Since then my N3DS won't turn on unless it's charged, battery problem I thought so I just bought a new one. When I installed the new battery I see the old battery become swollen, I thought nothing much until the new battery also become swollen just 1 week after installation. I didn't even use my DS since I've been busy with other stuff. There's something that broke that causes the battery to become swollen and I don't know what's that.

Any pointer is appreciated.