The reason for the heat of the battery when charging is: 1, the efficiency of converting electrical energy into chemical energy is 50%, and most of this 50% is released in the form of heat. 2, the battery has internal resistance, the less electricity the greater the internal resistance, according to P=I*I*R, the internal resistance consumption power is also converted into heat. 3, after the battery is full, if you continue to charge, the electric energy has almost no longer converted into chemical energy, almost all converted to heat, so the charger can not control the termination of charging, which is very damaging to the battery.