This works with my portable PC, and my 8000 mah USB battery. Yay! It doesn't seem to charge much if you use the GPS while plugged in unless it is in the car battery. To charge the GPS when the car is stopped, I found it charges nicely if you plug the GPS lighter adapter directly to the car battery with two little alligator clip wires.
Charging Garmin nuvi with generic USB charger. - YouTube
