During the battery charging process, electrical energy transforms into chemical energy, heat energy and other energy. It is normal phenomenon for that battery is heating during charging. But at high temperature, battery should be timely check whether charge current is too excessive or internal short circuit happen and so on. And, if there are less electrolyte, the internal resistance will increase, which also can cause the high temperature and high voltage when battery is charging. Aging battery, dry electrolyte, internal short circuit, etc. will cause heating as well. If charging equipment can't keep invariable at the later charging period, the voltage value will exceed permission value, and the temperature will raise at the same time. Finally, the battery will be bulged seriously and at the end of its life.
Try not to mean or upend the battery which is in use, for avoiding that lots of gas produced in the battery can’t exhaust from the valve smoothly, especially don’t do it when charging, or this activity will cause cover bombing.