Phone screen won't stay off when using external battery

So, I used my Google Pixel 3 when cycling for maps and sometimes when the battery runs low I use an external battery to keep the charge.

However, there seems to be a feature that when the phone is plugged into a power source, the screen comes on when you pick up or move the phone.

The problem when cycling is that it's usually bumpy and the keeps the phone screen on all the time.

Coupled with gps on, this means that my phone drains faster than it can charge.

I've tried to change settings to turn the screen off, but it always comes on.

Does anyone know a way I can fix this?

Thanks!

 

Thanks! That might be it!