This has only started happening in the last two or three weeks. At first it happened when my phone reached about 17-18% on the battery, but now it's the mid twenties. The phone will suddenly just turn itself saying it has run out of power. Yet when you plug it into the power it comes back on still saying there is about 15% left. It's running iOS 9.1 & not using battery saving. Is this a hardware or software fault?