Might be a dumb question: But why charger always heat up? Thanks !
Not a dumb question at all. Imagine the charger to be this conversion box :
The only problem being that the efficiency of this conversion from AC to DC is only ~ (80-85)%.
The rest of the energy is lost due to the resistance offered by the wires and transistors which gets reflected as heat or I2R loses. So, higher the resistance, more the loss.
Thanks for asking !