Over the last several weeks I have noticed that if my iPhone 6s battery reaches 33%, the phone will shut down and refuse to restart until I plug it in to recharge. Once it restarts, the battery will still show 33% and then continue to work without issue even when unplugged from power. This has happened even before I upgraded the phone to iOS 10. Is this a known problem? And if not, is it more likely to be a software problem or a hardware problem?