OK here is the deal...I researched this as it happened to me just that way a few days ago....but first, some background.
I bought the Samsung Galaxy 3 from radio Shack-My carrier is Sprint. The device has NOT been rooted. Had pretty good luck with the Samsung Epic (Galaxy S).
Installed: Juice Defender Pro-Almost DOUBLED my battery life until....
4 days ago, I awoke at about 6 am to see that the battery had been charged, so i disconnected the charger. Went back to sleep, and about 8am, was horrified to see that the battery had SOMEHOW drained to 23%...It had drained 3/4 of its capacity in just under 2 hours!
Well that aint happening. After some very tedious research, I was left with a few possibilities: 1) bad battery or device (no kidding) (2) Make some setting changes (i did that...I set my display to minimum and NOT to auto, and made a few other changes) But the thing that did the trick was uninstalling some apps that I just installed. Once that was done, the phone ran fine.
The problem is that some applications, especially third party ones, will not always work properly, and/or can drain the battery. The only way to know for sure is to uninstall them...wait to see if it helps, and if it does, then install them again, and see if the problem returns. i elected to NOT reinstall the applications. I figure that i can try others, and today, I only install one app at a time, and if the phone behaves properly, then it stays.
There are MANY things one can do to extend battery life. I LOVE Juice Defender Pro. It does what it says it will do, and was worth every dime I paid. There are other things you can do like Use airplane mode when listening to mp3's. I went on a motorcycle ride, listened to music for 3 hours, and only used about 8% of my battery life to do it.
Anyway, I hope this helps. Try getting rid of apps that were most recently installed before the issue occurred. It was the solution's for me.