How do you turn off the nightlight when using external power


I use the Edge 830 with the external battery option.
When riding in the dark, I don't want my screen lit up all the time as it kills my night vision (and drains the battery on long rides). Instead I prefer it to light up if I touch the screen or when a turn approaches.
When riding without the battery, this happens fine but as soon as I connect to the battery (and even if the battery is off), I no longer get the timer for the back light and it's lit up all the time.
Is there some way of overriding this or does riding with a battery mean it will just be on all the time.