Some heat during battery use is perfectly normal. However, unusually hot batteries could pose a problem if left untreated. Example causes are:
When rechargeable batteries are over-discharged with continuous high current, abnormal heat could be generated. If the batteries become excessively hot, stop the appliance immediately from running since it damages the batteries. After spontaneous cooling the batteries will be fit for use again.
When rechargeable batteries generate heat immediately after inserting them into an appliance, there is the possibility that the batteries will short circuit when contact is made with the appliance’s terminals. This could cause the batteries to leak and explode. If this happens, cover the batteries with a thick cloth immediately and remove them from the appliance quickly without scattering any of the leaking liquids. Then, let the batteries cool naturally in a safe place, after which you should put them in a plastic bag before depositing them into a recycling box at an electrical store.