The warning is the normal consequence of the CPU handling excessive data, which makes battery warm. It is a recommendation message and not harmful to battery or phone even if the phone feels hot.
You can follow below actions to make phone return to normal temperature:
- It is suggested not to expose your phone in the sunshine for long periods of time;
- Close large power consumption applications, such as music, chat applications, GPS applications, 3D gaming, video streaming etc.;
- Reduce the brightness of screen to lower the temperature