Glad i found this thread because my battery life has become an absolute joke ever since i updated to 8.1 originally.
When i first got my iPhone 6 and was on 8.0.2 or whatever it was, the battery was outstanding. Lasted me all day and i still had around 20-25% left at the end of the day.
I updated to 8.1 and BAM my battery drain is horrendous. I'd leave it in my locker at work during the day and only use it on a few breaks. I'd see it just drain so quickly. Thought i'd be patient and wait till 8.1.1, then upgraded and got even worse battery drain. It's disgusting, i've went to an Apple store and had an appointment with some guy, only for him to tell me that my battery is fine and that I should stop using certain apps ( the same apps I was using when i first got my iPhone 6 and had amazing battery life )
So tempted to demand a replacement because it has ruined my experience with this phone completely. I was loving it when I first got the phone and now I have to charge the phone halfway through the day. Got this phone on a 2 year plan and the battery doesn't even last half of what it used to.
Tried 8.1.2 and it's the same drain problems. I literally get 3 hours usage when I originally was hitting 6-7 on an average, light usage daily routine.
Really tempted to just give up with apple, sell my iPhone and just pay off the rest of my contract. I'm having to wait anxiously for every single update in hope that my battery goes back to how it should be!
Sorry for the long post!