This is one of the most talked about issues with the HTCS620 but it isn't the fault of the phone!
We've moved on so I've deleted all the references to earlier software versions that had the major battery issues. You should be running at least 2.43 by now
There are still a few issue that drain your battery
Close unnecessary programs. Windows Mobile 6 thinks you want to keep everything running so you have to find other ways of closing them. If anyone knows how to close a program without using applcloser or a Task Manager please let us all know. Surely there must be an equivalent to closing a window on a PC?
Switch wifi and bluetooth off when not in use
If you use the big battery & cover then insert the small battery it is liable to slip out of position. I just stuck a "non slip foot" from the pound shop to the back of the small one & it fits snugly. It also means you don't risk cracking he cover that isn't in use.
An excellent battery monitor is available here
Replacement batteries and chargers