Welcome to Android Central! The battery might be failing. Let the battery drain until the phone automatically shuts off, then plug the phone in to a wall outlet and let it charge overnight (while still powered off). Try to do this 2 or 3 times in a row, to see if it can force the phone's battery monitor to recalibrate. If it still has problems, then you may have to contact LG or a local repair shop for a replacement battery.