Hello. I have an iPhone 6S, and from day one it worked more or less flawless. Then, after IOS 9.2 update my battery endurance was terrible. I had to charge my iPhone almost twice a day. Sometimes also the phone turned off, even if the battery was at 50%, 30%... With the IOS 9.3 update, I was hoping that the battery issues will go away, but the problem was still there. So I was thinking to go to local Apple store, to claim the battery change (phone is now 6 months old). I was told, that Apple has very strict policy regarding battery replacement and that I will have hard time proving that the battery is fault. So before Apple store visit, I tried one more thing - DFU restore, not common restore phone and with setting the phone as new, and not restoration from backup with IOS 9.3.1. Result? Whoah! Now my iPhone battery lasts for 25 - 30 hours, without any issues, with normal usage. Of course I had to set all the settings and download every app again, but it was worth it. My phone works as new! Just wanted to share my solution to battery problem. So if someone else has also battery issues, maybe the DFU restore will fix it. For me it worked. Best regards!