Why does my Nexus One power off with 15% power remaining?
Probably your battery does not hold as much charge as it used to, and so your phone's estimate of 15% is incorrect; the battery is actually almost dead.  To get a more accurate reading, try the following:


  RECALIBRATION:
  
  A recalibration is mostly needed, when dealing with different kernels (ROOT!). Most custom recovery images provide the option "battery stats wipe" under the menue "Wipe".
  
  Here is how ya do it!
  
  
  Enter Recovery Mode
  do a full nandroid (or nandroid+ext) backup
  Enter "Wipe"-Menue
  do "Battery stats wipe"
  reboot
  Calibrate the battery by completely draining it until the phone completely shuts itself off. 
  Turn the phone on again and let it shut itself off one more time. 
  Then charge your phone while it is off for over 8 hours. 
  This will fully charge the battery so that when the Android is turned on, it now sees the battery as full.