there is an app called 'System Panel'. It has a ton of info, including battery temperature.

Java Programming

 

Hi;

it's normal very normal,that your phone gets hot when charging or when you're using certain apps for a long time.
and this is the scientific explanation of that Phenomenon
<< The exterior of the device functions as a cooling surface that transfers heat from inside the device to the cooler air outside.>>

but if you do get The temperature warning message, then you should turn your phone off and move it to a cooler place and let it cool down before you try to use it again.