I feel like I may be overthinking this but: I want to use my mobile hf radio while the car is off, using a separate lifepo battery for power (not the car's battery). It's a mag mount antenna. My understanding is that such antennas are sending the center conductor of the coax into the arial, the shield/ground into the car body. It's my understanding then that assuming the car is otherwise well rf "grounded" I wouldn't have to run a wire from the battery ground to the car body to ground it, i.e. that should happen through the antenna mount. That said, there's still the matter of the car battery's negative terminal, which is now through the car's body in contact with the lifepo battery's negative terminal, if I understand correctly. However, to reiterate, the car battery's positive terminal isn't part of the circuit. Part of me thinks I can ignore the car battery as not being part of the circuit. The other part is concerned about what if there's a voltage difference between the two negative terminals, i.e. the car battery's negative terminal and the lifepo battery's negative terminal. So first question is, can I just ignore the car battery when the car is off? Second question, what if I want to operate mobile this way with the car running, i.e. just run off independent battery power?