I still have GB 2.3.6 but also experienced short battery life. Sometimes I would be fully charged in the morning but by noon my level was down to 50% or less. I couldn't go all day without charging it. I always had to plug it in at my desk at work and in my car so the battery wouldn't die.
I bought a 3800mA extended battery and now I usually have a >50% charge after 12 hours! I never plug it in all day and find it difficult to drain the battery in a single day. With the extended battery and the cover the phone is 5mm thicker over the battery compartment, total thickness 15mm. That is really not too bad. My old Palm Centro was 18mm thick stock, and that was a small phone. I'm very pleased with this extended battery but some people love a thin phone so this might not work.
I hope I don't sound like a salesman 'cause I'm not. This is the extended battery I bought but there are many others like it.
Trexcell Samsung Epic 4G Touch 3800mAh Extended Battery Cover Sprint Only | eBay