Long story short, my brother in law broke his 5S and I gave him my old 6. Now he's saying that the phone randomly turns off when he's doing things and shows a low battery indicator, even when it was at above 20%. I've never had this issue when I had the phone, so I'm wondering what might be going on? I can replace the battery but I can't imagine that it just randomly broke when I gave it to him? There must be some correlation. When I brought it to him it was on 9.3.5, but had to update it to iOS10 (whatever is latest) I also installed all of his backup. I would be more inclined to think this is a software bug than a hardware thing, but has anyone else been experiencing this? Thank you.