There has to be a reason for the battery drain.
I have an 820 on DP 8.1, and was getting approx 26 hours. On the 6th Dec '14, I got a critical update which I downloaded.
Battery life diminished to 12/13hours after this 'critical' update.
If I use location and 3G the battery life is less than 8 hours
Did a hard reset (clean, not from a backup) and battery life back to 26/28 hours. Did another hard reset and used most recent backup (to get my call logs back) and battery life dreadful again.
I am a software engineer in the telecommunications sector (not phone OS dev) and if our software had the same effect on our customers hardware we wouldn't last a day.
There is a reason for this battery problem and it's down to bad software engineering.
[rant over] anyone got a proper (tried & tested) solution ?