electrode

I recently got a new battery for my MZ-HN900 (from minidisco), and I drained it completely, then charged it in the cradle... I left it in there about 5 hours and when i came back i tried to get it out and it was stuck, so i pulled it out with some pliers, to find out that it had doubled in size!! My Minidisc player still works with the other battery (luckily). Has this happened to anyone else? Is this a defect with the battery, player, cradle, or me?