Has anyone come up with a way to charge the Rino 750/755 with just a USB cable (not the wall outlet)? Last year we used a car jumper pack with AC plugs and the wall charger, worked fine for base camping but I would really like to be able to just charge off of an external battery pack. I have tried just using a standard usb cable but it will not charge from a usb plug, I’m guessing it is an amperage issue but electricity is PFM to me.