The most common cause for laptop overheating is due to the lack of heat transfer, which is because there is no air flow in the vents of the laptop. An accumulation of dust in the vents and fans through the cooling channels might be the main reason, blocking the air flow and therefore causing the laptop to overheat.

Try unplugging the laptop and letting the battery to die completely, then plug it back in and leave it to charge fully without using the laptop. If it is still losing charge quickly then the battery is most likely bad and must be replaced with a new one.