If you're a rooted user, chances are that you flash new ROMs fairly often. What you may not know, however, is that your phone saves the battery statistics from old ROMs, and if you've never recalibrated your battery before, then your time away from the charger may not be as good as it could be.
Resetting battery stats can be an intimidating task for those who don't spend a lot of time in recovery (or who have never even booted into recovery). Fortunately, XDA member marosige has created an app that makes battery calibration a snap.
It's really simple (the instructions are even clearly stated within the app) - launch the app, charge your phone to 100%, unplug it, and tap the "Battery Calibration" button. The recalibration process will begin at this point, and generally takes a few days to complete. Use your phone normally and enjoy the additional battery life!
As always, check out the support thread in the XDA Forums if you have any questions.