Serious battery drain (Screen, Android OS) after JB upgrade Since I upgraded my Samsung Galaxy SII from Android 4.0.4 (ICS) to 4.1.2 (JB), my battery life has decreased dramtically.

Running ICS I could easily have ~25% battery left when going to bed, after about 20 hours of moderate use. 
That is with a few phone calls, reading work/private e-mails as they arrive, occasional news reading, internet browsing and Facebook.

My usage pattern is largely unchanged, but since the moment I upgraded to Jelly Bean, I can barely make it through a working day without charging.

Looking at earlier battery stat screenshots (I'm a nerd, guilty as charged), my phone would use about 2-3,3%/hour. Now I see usage of between 5,7% and 10,6% per hour!

The top power consumer is, as always, the screen, which takes up 35-45% of the power used - just like before. Only now, it has drained the same amount of power, 2-4 times faster - or about 1% of the total battery capacity each minute it's been on.

  

(click images to enlarge)

72% of the total capacity used, of which screen consumed 38% (relative) or 27% (absolute) 

Android OS has moved up the list of power consumers. Now using typically 16-17% (relative) or 12-17% (absolute), as opposed to the previous 9% (relative) or 5% (absolute).

I never switch off WiFi completely, unlike GPS which is switched off (thanks, Facebook) unless I actively need it. Display is set to auto-brightness - just like before.

I have completely drained, then fully re-charged, the battery at least three times, to re-calibrate the battery gauge. It didn't help my battery life.

Any idea of what's going on here and how to fix it (short of doing a factory reset)? How can the screen suddenly drain the power so fast? What's causing Android OS to burn more fuel as well?
You are suffering the same problem I was after upgrading. Look in the direction of either Google Now or Google Maps. Good chance you have 'Report from this device' set in Maps Location Settings. This is murder on your battery! I disabled it and am now back at about 2% - 3% per hour on 3g with light use throughout the day. I also disabled Google Now for good measure, but I didn't notice any real improvements from that.

I do still notice much higher battery consumption than 4.04 when having wifi on. At home, I have reverted to 3G after seeing it burn through the battery at 5% per hour even without any active use.