From the Home screen, tap the Menu Key > System settings > General tab > Battery.
The battery level (as a percentage of fully charged) and the battery status (Charging or Discharging) is displayed at the top of the screen.
Checkmark the Battery percentage on status bar checkbox to display the percentage of remaining battery charge on the Status Bar next to the battery icon.
Tap Battery saver to view and/or modify your Battery saver settings.