Own made battery was melted💢

I bought batteries and a battery case which is compatible with the Makita BL1860.

I do some welding jobs and soldering jobs. I used it a few times and recharged also.

While charging the battery, I heard some small explosion sounds. The sound like capacitor was damaged.

It was hot and melted….

I’m not sure why it was melted.

I try to make again~