For a couple of days my daughter's 6s has been stuck in a restart loop. It tries to boot up, gets to the white screen with the apple logo, screen goes black and then it cuts off to try again. I can't restore it either normally or by DFU and am getting the old iTunes error 21. I've also tried all combinations of home button/power button/volume button but no joy. She mentioned it wasn't charging properly a few weeks back but when I tried it with a different cable it seemed to be fine so I thought it was something she was doing. Now I'm thinking that the phone is trying to power on but can't draw enough power from the battery as it's loading the OS and that's causing it to cut out. As I type this I'm convincing myself it's the battery but is there any way to test it before I spend £20 on the kit? I suspect not but just thought I'd ask.