Battery saver mode
Battery saver mode changes certain settings to conserve battery power until you can recharge your phone. If you turn Battery saver mode On, it will automatically activate when the battery charge level drops to the level you set. If Battery saver mode is off, you’ll be prompted to turn on Battery saver mode when the battery reaches low levels.
From the Home screen, tap the Menu Key
> System settings > General tab
> Battery > Battery saver > Turn Battery saver on.Select the option you want. Choose from Immediately, when the charge drops to 10%, 20%, 30%, and 50%.
Checkmark each battery saving item you want to include. You can set Auto-sync, Wi-Fi, Bluetooth, Vibrate on touch, Brightness, Screen timeout, Front touch key light, and Home button LED.
Tap the switch
at the top of the screen to turn it on.
You can also activate Battery saver mode using the Status
Bar. Open the Notifications panel, then tap the Battery saver icon 
in the Quick settings
to toggle it on or off.