There are several tips from which you can save battery on your iPhone if you are in a position where you wont be getting the charger for a reasonable amount of time.

• Switch on the Low Power Mode

Go to Settings > Battery > Select Low Power Mode On

• Turn off the colors

Go to Settings > General > Accessibility > Display Accommodation > Color Filters > Switch it on and Select Grayscale.

This saves a considerable amount of battery

• Turn off Location Settings

Go to Settings>Privacy>Location Services.Turn it OFF

N.B Turning Off Mobile Data also helps reducing battery life but then it may be essential to users. (LTE/4G draws a lot of battery)