I found that in their getting started iOS (https://support.pix4d.com/hc/en-us/articles/204692015–iOS-Pix4Dcapture-Getting-Started)
" Battery : As the battery lifetime depends on multiple factors such as wind, altitude, temperature, etc., it is impossible to predict the lifetime of the battery. If the app is connected to the drone and the battery is under 20%, the drone flies home. If the connection to the drone is broken and the battery is under 15%, the drone lands where it is. Manual control might be required at that stage."
I think the best would be to do some tests in the field when the drone has not much battery, to see how it reacts. For example by setting a small mission when battery is already low.