Ah, this is a very common issue regarding the battery state. If this happens, the first thing to do is replace the battery. Either it became defective over time, or the battery is blown out. Do not do anything to overheat the phone, as if the problem could be a leak in the battery, and the phone could explode. Android batteries are very cheap, and it is super easy to install, in fact they are much easier to install than apple batteries.
Happy fixing!
-Toby