Battery saver changes certain settings to conserve battery power until you can recharge your phone. If you turn Battery saver On, it will automatically activate when the battery charge level drops to the level you set. If Battery saver is off, you’ll be prompted to turn on Battery saver when the battery reaches low levels.
Tap the Home Button > touch and hold the Recent Apps Button > Settings > Battery > Battery saver.
From the Battery saver screen, select to activate Battery saver immediately, or when the charge drops to 5% or 15%. If you turn off Battery saver, when the battery charge becomes low, a notification prompts you to turn it on.
After you modify the Battery saver settings to your preferences, tap the switch to toggle it On (blue) or Off (gray).