Well, depending on how old the battery is, it could just have reached its limit. Meaning you need to replace it. Or it could be a flawed battery if it is quite new.
If the device is under warranty I would contact the manufacturer about the issue and have them fix it. If, however, it is no longer under warranty they you can either do the work yourself (if you feel comfortable doing so) or take it to a local tech.
I would do this soon though. As the overheating can fry components and make the device totally unusable.