Source: Chris Hoffman, HowToGeek.com, September 30, 2017
Your iPhone has a “Low Power Mode”, which you’ll be prompted to activate it when your phone reaches 20% battery.
However, you can enable it before that point to stretch your battery life further. Starting with iOS 11, you can also activate Low Power Mode right from the Control Center.
Low Power Mode disables settings like mail fetch, Hey Siri, and other things that people commonly change when they want to make their iPhones live longer between charges.
