XDA-Developers Forums:
Hi *******, I understand your concern regarding battery life on your Nexus One device. The following steps should significantly extend the battery life on your phone. Please connect the phone to the charger with the phone powered on, and allow the phone to charge until the notification LED is green, indicating the device is fully charged. Disconnect the phone from the charger, and power it off. Reconnect the phone to the charger with the phone powered off, and allow the phone to charge until the notification LED is green. Disconnect the phone from the charger and power it on. Once the phone is powered completely on, power it off again and reconnect it to the charger until the notification LED is green. Disconnect the phone, power it on, and use it. You need to use this sequence only once. If the issue of battery life on our phone persists, I recommend you contact our HTC accessory department directly.
So to break those steps down
1. Connect to the AC charger with the phone on, until the LED is green
2. Disconnect the phone from the charger, and turn it off.
3. Reconnect the phone to the charger, while it is off, and charge it till the LED is green.
4. Disconnect the phone from the charger again, and power it on.
5. Once its completely on, turn it off again, and connect it to the charger once again till the LED is green.
6 (Optional). Extra step for rooted users here! Disconnect the phone and boot into recovery mode and select to wipe battery stats. This will help with the calibration process. Additionally, after doing this you may want to repeat the powered off charge again, this boosted my battery a little more.
7. Disconnect the phone (if you didnt already) power it on, and use it.
If its anything like mine, you'll be amazed at how much it charges further than when the LED was showing originally green when it was on, and then again when you do the second powered off charge.
If it goes green near instantly, then your battery doesn't require calibration, but doing this process can't hurt.
If you type in *#*#INFO#*#* into your keypad, then go to battery information, the maximum the battery can be is 4200mV, which indicates a totally charged battery. Chances are no one will get this high, but it should at least get pretty close (above 4100). Mine got to 4182.
Apparently you should do this after every ROM flash also, and the main cause of this being required is flashing while a USB cable is in which messes with battery stats