Battery saver helps extend battery life until your next charge. Your phone can prompt you to activate it when the battery is low, or you can enable it anytime.
When the phone is in Battery saver mode, you'll see 
To save power, your phone will:
Open quick settings and tap 
To turn Battery saver on when your battery is below a specified level:

To prevent your phone from turning Battery saver on before you need it, you can set it to turn off when your phone is charged to 90% or above:

