The iPhone comes with a power save feature that extends the battery for times when you are not able to charge the phone. Sporting events, gatherings, camping trips, etc are some examples where you can extend the power your phone battery has by activating “Low Power Mode.”
Go to Settings, then Battery and slide to turn on Low Power Mode. That will stop mail fetch, refresh of apps, automatic updates and some of the visual effects. Another option to change is the brightness of the screen. That option is also in Settings under the option Display and Brightness. Slide it to less than fifty percent to really save power.
When you are able to charge the phone, the Low Power Mode should deactivate, however, you will need to raise the brightness if you want to brighten your screen again.