I think your battery is exhausted, it must have several years.
This is the problem with the current battery portable equipment.
Start with change it and GPS Magellan will found a good autonomy.
I don't think your USB cable has a problem.
You can load the Magellan GPS with the computer, but on the car with the charger connected only to the cigarette lighter.
The USB socket of the vehicle is not designed for this, just for a USB unit that consumes little.
On another forum, someone put down his RT6 wanting to load a GoPro camera like that.
Some equipment may have a special USB cable wiring for running correctly.
I had on the old car a dashcan that had this connection.
With a standard cable, she was in charge, but impossible to use it.