1. Use the mobile app to power off the pi. This will safely shutdown the raspberry pi. This is the best practice but from our experience, the pi should work fine even it is not shutdown by a command.
2. Disconnect the battery from the MM1. 

Note: The system will not be shutdown if you just turn off the Pi. MM1 will still draw power from the battery until the battery is drained. Make sure you disconnect the battery from MM1 after you finished using the car.