The iPhone has combined at least seven backpacking tools: maps, cameras, compass, gps tracking, satellite messaging and, in an emergency, your headlamp or flashlight. It's more important than ever to make sure your phone doesn't die mid backpacking trip. These tips will help you save your battery but you should always bring backups, especially a paper map. Most of these tips will also work for Android.
These battery saving tips are for Apple iOS 11 but will likely work for other iOS versions.
8 Tips Listed in Order of Importance
1) Use Airplane Mode Seems obvious but most people don't realize that location-based apps like Maps, Strava, GAIA GPS will still track you in Airplane Mode. Which leads us to:
2)Turn off Location Services for Apps you are not using Path: Settings/Privacy/Location Services. Many apps will "Always" use your location, so turn them to "Never" or "While Using". To truly save battery, turn off Location Services for all apps.
3) Lock your screen when not in use. Obvious, but often overlooked. Make sure your phone auto-locks ever few minutes.
4) Close All Apps Your Not Using
5) Turn down screen brightness
6) Turn off background app refresh Path: Settings/General/Background App Refresh. Do this for everything except the app you are using. Very important: do not do this for the GPS app you are using to navigate. If you do, you may stop recording your track and therefor not have a clear way to get back to your starting point in an emergency. Test this out before your trip.
7) If you don't go into Airplane Mode, you need to manually turn off a few things: Bluetooth, Cellular Data, and Wifi and turn on Battery Saver mode.