I believe the discharge current is
calculated by determining what hardware is on and how much the CPU is being used. It is just too consistent to be a value that is physically measured/sensed.
I have a simple utility (attached) that can display the discharge current when you ask for it, as opposed to continuously. With the radios off and the backlight on the dimmest setting, my battery current is always 57 mA. One increment in brightness adds 9 mA. With the nuePower/Backlight CPL utility, however, the battery current becomes exactly 100 mA higher because of the continuous polling.
If you take your battery's mAh rating (900 for the GSM version, 1340 for the CDMA version) and divide it by the disharge current, you should obtain your runtime in hours. I've tested my battery several times and found that it actually lasts a little longer than it should, even though I'm not particularly happy with my battery life.
I think I may have found another reason for the poor battery life. Please see "Issue #2" in
this thread of mine.