Iphone 6s fault

Ok. Not a console but someone might of had same issue.
Iphone 6s A1688. Turns on fine and charges fine. All good. Until it shuts off with no warning. No shutdown just simply dies. Plug into charger and nothing happens. Add a usb volt meter and the meter shuts off like its being shorted.
Leave the phone unplugged for a while and it will start and charge like normal untill it shuts off again, 5 mins to 2hours later.
Any one have any ideas? Thinking charger dock rather then tristar atm but still unsure.