Battery drains after 8.1.3 upgrade on my iPhone 6.

Try to reset the phone by holding the sleep and home button for about 10sec, until the Apple logo comes back again. You will not lose any data by resetting, but it can cure some glitches.

If this does not help, setting it up as new device would be the next step:

How to erase your iOS device and then set it up as a new device or restore it from backups

Maybe also take a look at this page about battery drainage before setting it up as new device.

This does not help... iOS 8.1.3 battery life is terrible. While being on 8.1.2 i was acutally saying that Iphone 6's battery is great i had like 1h 30 min usage per 10% Now i get only 2 h usage per 30%

There were some other similar posts (that I can not find now) with a suggested solution that worked for me on my iPhone 5. Before 8.1.3 my battery lasted all day easily, after 8.1.3 it dropped like rock. The posts suggested some kind of infinite loop retry related to the iPhone trying to access an account (the ones under Settings > Mail,Contacts,Calendars > ACCOUNTS). The suggested solution was to log into https://appleid.apple.com/ (I did this from a laptop) and update the password if prompted to, because the current password was not secure enough. I logged into my apple id and was not prompted because my password was secure enough. I then logged into my wife's apple id (that I access from my phone) and was prompted for a more secure password (bingo). I made this change last night and my iPhone 5 battery is back to normal; it is like day and night. I guess it's possible Apple fixed something on their end at the same time; whatever, it working great now!