How do I enable battery saver on Phone (2a)?

1. When your phone's battery is below 20%, a low-battery notification will appear to prompt you to turn on the battery saver.
2. To toggle it manually, swipe down from the top of the screen two times to access the expanded quick settings. Press the edit (pencil) icon and drag the Battery saver tile up from the hidden tiles. Click it to toggle it on and off from the quick setting.