The Mission Control app allows you to chose how to draw power from the batteries when using a Range Extender. We recommend sticking to the default parallel battery discharge scenario when using a Range Extender battery. This way, the motor will perform better throughout your ride and neither battery will fully depleted unless necessary. Discharging the Range Extender first only makes sense if you want to remove it at some point to have a lighter bike or if you want to make room for a water bottle.