I used my Macbook 13" Unibody (got it a couple of months before they turned pro) for gaming and torrents so it was on a lot, one night a loud bang noise awoke me, turns out the battery swelled up, thankfully it didn't explode.
Was it related? no idea, i got a new battery (had to buy it and it was still under applecare, sometimes i really hate not having a direct apple store down here) and sold it to a friend, it's been solid (except the hinges have lost a bit of their strength) ever since.
Battery has nothing to do with this....if the cells are not sealed correctly you can get these kind of problems; and batteries (especially the ones made in china) are more prone to be defective, compared to any other component.