What ROM/kernel are you running? I have had terrible battery life since switching to AOKP a few months ago. With almost no phone usage, wifi & mobile data off most of the day, my battery might last 10-12 hours from a full charge. It used to go 2 days on the stock GB ROM. I am running AOKP 40 w/ Devil 0.99. There has been some discussion of battery life in the AOKP thread, but every time someone brings it up, everyone jumps on them and tells them it is just their usage habits that cause short battery life. I don't believe that, as I do not use my phone much all day while at work. The battery stats say that the Android OS is by far the biggest consumer of battery. That tells me it is the ROM or kernel. I am not using LiveOC or doing anything fancy with the settings. Also, I am using an 1800mAh battery already. Other than trying a newer version of the ROM and/or kernel, what can I do? I had trouble restoring apps when I tried to flash AOKP MS6, so I just stuck with 40 because I don't have any problems other than poor battery life.
I think you misunderstood whats being said...my fascinate gets good battery life while powered on. However, since upgrading to the gs3, my fascinate discharges a full charge in a day when powered off(Never knew this because I never left it off for a full day before). Pretty sure Tonu has the same issue.