Then you've got no worries. Don't connect anything to the car's electrical system. If the chassis of the radio, or the shield of the antenna cable, or the negative lead of the (stand-alone) battery happen to touch the car body, you're ok, but such a connection isn't required. The base of the mag-mount will capacitavely couple with the car roof, but this won't provide a DC path. You're describing something equivalent to using an HT in a car, with a mag mount antenna. Lots of people do that.