GPSMap66 Battery Charging

[Cheese_Spread_Butties]

Can the GPSMap66 be charged from a portable power bank while it is being used for navigating, or can it only be charged while switched off?

There is this information on Garmin's website:
https://support.garmin.com/en-GB/?faq=BW0eAHq4dhAH1rrKx69Vm8&searchQuery=external%20battery

This does not say that charging while in use won't work.  It just says charging time is best with the unit switched off.

I understand that the unit would not be weather proof while doing this with the connection cover off.