Android phone discharges when it has 20% Possible Duplicate:
  Why does my Nexus One power off with 15% power remaining?  




I've noticed that when my Google Nexus One phone is showing ~20% of the battery, after using it for a minute or two it completely discharges to 0%. Everything started before two weeks or so. Before that phone was fine, but now if it's showing ~20% of battery, after a few minutes it turns off and when you turn it on, it shows that there are 0% left in a battery and turns off... Where's the problem?
There are rumours on bad battery stats being able to cause this, and recommend "battery calibration". If your device is rooted, you could try that (it certainly cannot hurt or cause damage) -- though other sources doubt that theory, there are several apps on the market to help you with the calibration process (e.g. Battery Calibration).

Background: The system learns about the battery state, to understand it better with the time. So if it learned wrong, it might think "hm, battery should still be good, 20%" -- while it's already empty. Then the battery suddenly does not provide enough juice -- and the phone turns off.

It also works the other way around, according to reports, where the device kept running for 2 hours on 1% charge...

Edit: You might also want to check battery - Why does my Nexus One power off with 15% power remaining? - Android Enthusiasts