How is the U.S. President's cell phone kept charged?Not entirely sure why I was aksed to answer but I will give my best speculation on the matter. I have zero direct knowledge.
Likely there are spare batteries carried along by his staff or secret service agents which are charged using a direct battery charger and then swapped in when needed. Of course let us not forget that the president is unlikely to ever be out of contact due to redundant communication methods.
For example this device is used to charge blackberry batteries outside of the phone.