I have had my Iphone 6 for a year and a half now and it randomly shuts off. I have to have it plugged in constantly. Sometimes it will act up and have a contionious cycle of shutting off and turning back on, showing the lock screen for 2 seconds, then saying its on 1% then shuts off again. There shouldn't be any water damage, I have dropped it a few times but nothing enough to damage it. Why is it doing this?
