Your iPhone has a “Low Power Mode”, which you'll be prompted to activate it when your phone reaches 20% battery. There will be a popup to remind you, here’s the tutorial to customize the content of this popup.
After we jailbreak 32-bit iDevice, we are able to enter KDFU mode and then downgrade back to iOS 8.4.1 using 3uTools. Here’s the guide for those users who want iOS 8.4.1 back.