i have encountered a bizarre problem. My iPhone 6 battery started draining rapidly about a week ago. Within 3-4 hours from 100 to 30 percent. I have done extensive troubleshooting, including all the battery saving tips to no avail. I have been to the Apple Store and we determined that the hardware is all ok. After additional troubleshooting I determined that when I delete my iCloud account the problem goes away. This led me to investigate my iPad air 2 as well and sure enough same problem there (the battery is a lot larger and I did not notice it before).
When I look at the battery usage in settings both standby and usage time are identical when iCloud is signed in. When I sign out of my iCloud account the usage and standby work as they should. i subsequently tried turning off individual services with iCloud (ie mail, calendar, reminders, etc.) sequentially but the problem persists. Only signing out of the account completely resolves the issue. Apple Store has escalated it to engineering and I am waiting. Was wondering if anyone had any suggestions. This sounds like the .mac problems all over again.