Will source a new internal battery. Seems you can not attache an external battery to the GPS
I have a friend who has an external battery for his cell phone. It also has the same USB plug as my 3590 has. I haven't tried it, but I'll bet ya a nickle it'l work as his cell phone 120v charger will charge up my 3590. Both put out the proper voltage, and the amps doesn't matter.