My iphone 5, was a secondhand device bought a while ago as a replacement to other model i have had. It has the latest ios on it. It was working ok despite it came in contact with water and then was cleaned with isopropanol and started up ok. The charging dock was also replaced from another iphone all were ok. I remember when phone was charging with usb went upto 86% then stopped charging. When i re-mounted the phone to charge it to see if goes tupto 100% it worked. The phone possibly shutdown to approx 5 Celcious. Next morning i tried to use power button when i saw it was off. It did not startup at all. I decided to use the Restart or force quit combination (pressing power button and home button). Phone started ok and since then i have no problem, but i think it will shut down again if battery is left to low percentage and will not startup. Any clues where the problem is please.