The device referred to above could take your GPS up to 5.2 volts, depending on the usb adapter, Some but not all rechargeable batterys are 1.2 volts, but you need to check this.
Worst case scenario is your voltage is over by 1.6 volts, electronic gear like yours is more sensitive, and expensive, you can buy regulated cigarette lighter socket multi voltage adapters more suited to your needs.