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 will be prompted to turn on Battery saver mode when the battery reaches low levels.
From the Home screen, tap the Menu key > Settings > 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%.
Tap the switch at the top of the screen to turn the feature on.