Battery saver helps extend battery life until your next charge. Your phone can prompt you to activate it when the battery is low, but you can turn it on manually at any time or set a schedule to turn it on.
When the phone is in Battery saver mode, you'll see 
To save power, your phone will:
Battery saver turns off while your phone is charging. It turns back on when you unplug the phone.
Open quick settings and tap 
You can set Battery saver to turn on automatically based on a schedule or when the battery reaches a specific percentage:

To turn Battery saver on and off at certain times:

To turn Battery saver on when your battery is below a specified level:
