This is precisely why I cannot push the "install" button from the JellyBean update notification that I receive on my phone about 30+ times per day. Yes, I get a push notification that many times and more daily from Samsung to take this update and I just cannot do it. I was a happy owner of a GS2 running GingerBread and happily took the ICS update when it came out. With that update, I got a max of 4 HOURS on a phone that used to last well over a day with moderate use and processes running in the background.
To try to get the battery to last longer, I had to reflash ROM, do a factory reset, cycle the battery, turn off all processes and background updates, and still only got about 5 hours from my phone. I finally broke down and upgraded, at a pretty penny, to the S3 and now I read all of the horror stories about the upgrade to JB and I just cannot believe that Samsung, Google and Sprint don't do extensive beta/field testing before these roll outs and offer some sort of way to revert back to the OS you were using if things go south.
I would have happily taken back gingerbread with my S2, a phone that in some ways I liked more than my S3, mainly the brilliant display it had. The S3's display may be slightly finer resolution, but it is washed out compared to the S2.
Okay, I hope you try a couple of the other things offered by people here to get your battery life back to what it was. It stinks to lose the "mobile" part of mobile phone...