I average 10 hours of battery life on my iPhone 6 and this is with heavy web browsing, gaming, and video watching. The only issue is that 2 days in a row, my battery went down by 15% overnight. What could be causing this issue? This happened ever since iOS 8 because I never had this problem on my iPhone 5 with iOS 7. I would lose maybe 3% tops in sleep mode overnihgt.