iPhone 6 will not turn on after being left on overnight.

We have a recent client that has been having this issue reoccur. When he gave us the phone it would not respond to anything. we tried plugging into a PC to connect using iTunes with no success, would not show a red low battery when pressing the sleep/wake button, no charging indicator when plugged into charger, Home + Power held for 30+ seconds with nothing. So after opening up the phone and testing a new battery on it, it booted right up. So I charged the battery to full and left it on overnight ( off the charger) to see if it would just randomly turn off and see if it would do it again. I come in this morning and the phone is back to doing what it was doing before. Does anyone have any insight on what may be causing this issue.

P.S. The phone has was factory reset so i doubt it would be a software issue that is causing this. All answers will be appreciated!