"Independent" battery power in a car

Discussion in 'On the Road' started by KN4YRM, Jul 8, 2020.

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  1. KN4YRM

    KN4YRM XML Subscriber QRZ Page

    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?
  2. KC0LDH

    KC0LDH Premium Subscriber QRZ Page

    I belive you are on the right track being concerned about the voltage difference. My vehicles both have dc shunts for measuring power installed on the negative terminal. How would you wire the battery to the ham gear? Will it be charged from the car when running? Will it be connected to the OEM 12v system at all?
  3. KB3WFV

    KB3WFV Premium Subscriber QRZ Page

    How is the lifepo battery going to be charged?
    It must be mentioned that a lead acid car battery and a lifepo battery are charged completely differently. So, connecting the two battery together (positive and negative in parallel ) would require an battery charge isolator that is designed to accommodate the combination of a lead acid battery and lifepo battery. The post indicated that only the neafative battery cable would be connected between the lead acid and the lifepo. So, how is the lifepo charged ?

    To answer your questions. I would use the lifepo battery completely independant of any connection to the car, negative cable or otherwise. The RF return path to ground is the metal of the car and it's inductive coupling to the magnetic base of the antenna to the coax shield and finally to the radio . The radio power is connected appropriately with fused connections to the positive and negative terminals of the lifepo battery. That's it.

    To switch between the car battery while mobile and the lifepo battery while stationary I would use fused Anderson power pole connections. When mobile plug the radio into a power pole pole connection at a power source from the car. When stationary unplug from the car and plug into a power pole connector on the lifepo battery. Always turn off the radio before unplugging or starting the car.

    This website may be of some help to your mobile endeavors.....



  4. KN4YRM

    KN4YRM XML Subscriber QRZ Page

    The lifepo will not be (intentionally) connected to the car wiring at all. I'll take it inside the house when not operating and charge it in the house.

    (Bigger picture is I'm going to operate remote from inside the house on rainy/lightning days, just using the car as a "dry box" essentially, and I don't want to use the car's battery at all).
  5. KD0CAC

    KD0CAC Ham Member QRZ Page

    Your idea / ideas are so messed up , now I'm confused ;(

    1st off RF & DC grounds are 2 different things .

    " it's a mag mount antenna " - the mag-mount is one thing / the antenna is another , what antenna - make & model are you referring to ?

    The coax has nothing to do with the 2nd battery or DC power to radio - or at least in the sense that using the coax shield for power ground .
    And that brings in the idea that the mag-mount grounds to car , the closest is that the mag-mount antenna [ depending on the type of antenna ] dipole or or the antenna in question is one half the antenna and the metal of the car acts as a ground plane for the antenna ?

    How do you figure the DC power is getting through the paint on the car ?

    Then in the future , so you can charge the second battery , using something like a battery-isolator - this allows the alternator to look at each battery separately so they will both get charging current .
    Which brings to a big safty issue , more details on the 2nd battery , make & model ---- does it have a BMS - Battery Management System , if not you need to add that -- no matter what / where the battery gets charged , car or house .

    So then , well lets see back to some info related to your idea of the antenna/mount being part of the battery negative / ground - not connect .
    Some basic electrical concepts - DC - direct current requires 2 wires because positive voltage requires one wire and the negative requires another wire .
    A battery with only one wire is basically turned off , and both wires are a single circuit .

    AC alternating current - both the pos. & neg. are on one wire , for some devices , that's all you need - some lites , some will be dimmer on one wire .
    Then if you need to run other devices like a motor - it needs a second wire - neutral = the neutral returns to power source , grid or generator etc.

    Both of those are circuits , think of a circuit as a circle / loop and loads & switches interrupt/or are part of the circuit ,
    generally , DC loads are parallel and AC loads are series circuits , there are variations but to start with grasping concepts .

    Then as asked in the 1st reply , how do you plan to charge the 2nd battery ?
    AA5LS and K0UO like this.
  6. KN4YRM

    KN4YRM XML Subscriber QRZ Page

    1. Lifepo will be brought inside house and charged there. (basically I want to use my car as a remote station on rainy/lightning days and want to use a lifepo instead of the car battery).

    2. I could I suppose disconnect the car battery's negative terminal altogether when I operate the radio from the lifepo. Kind of an annoyance, which I'm wondering if it's even necessary as the car battery's positive terminal is not part of any circuit here.

    3. As for your power plug suggestion, that's exactly what I'm thinking. However, it seems to me that the car battery's negative pole is tied to the car body (unless I disconnect it) even when the radio is plugged only into the lifepo for power, and this post is about my wondering if that matters at all.
  7. KN4YRM

    KN4YRM XML Subscriber QRZ Page

    OK your questions helped a lot. To be clear the radio will be plugged only into the lifepo battery. Lifepo will never be charged by the car in any way.

    First off charging to clarify I don't plan to charge the lifepo using the car at all. I'll disconnect it from the radio and bring it into the house to charge.

    It's an MFJ-335BT into which I'll screw a 20m hamstick.

    I agree there is no DC path between the two batteries.

    My experience with dipoles and ground planes I've made is the shield gets soldered to one of the dipole side or to the ground plane radials. I just assumed that somehow the magmount is coupling the shield line to the car, either direct touching or capacitive coupling or who knows.

    I just got to thinking that the radio operating off lifepo is putting some kind of voltage into the coax shield, which through mount then through the car body ultimately goes to the car battery's negative terminal.

    So I could still be wrong about what's happening, or right and need not worry about it, etc. I guess it comes down to whether it is really necessary to disconnect the car battery (or at least the negative terminal) or if it is inconsequential in this setup.
    K0UO likes this.
  8. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    Why do you want to run two batteries in the first place???
    A mag mount on HF is not a great idea unless you run another ground to the body anyway.
    Even running a 100 watts on sideband, CW or digital, a car battery will run many hours even with a high duty cycle.
    I suggest that you keep your battery above 12 volts because some radios behave poorly at anything less, that's why I would run a battery isolator if I was going to run two batteries to keep the radio's battery voltage up.

    I strongly suggest you review Alan's web site @ k0bg.com
    Last edited: Jul 8, 2020
  9. KN4YRM

    KN4YRM XML Subscriber QRZ Page

    I am not thinking of it as running two batteries. On rainy days I plan to operate the mobile remotely, it will be in my car in my driveway me inside, this way no antenna feeds to the house/shack. I just want to unplug the mobile from car power, plug into lifepo (lifepo also powers the raspberry pi that allows me to get in via wifi). When I'm done I go retrieve the lifepo and bring it inside to charge, hook mobile rig back up to car power.

    I do not want to use car's battery because I don't want to be in position where I've drained it and can't start the car, etc.

    Think of the car as just a "dry box".
  10. K7JEM

    K7JEM Ham Member QRZ Page

    I would tie the frame of the radio to the chassis or frame of the car (through a short jumper), just to make sure those two are at the same RF potential.
    KN4YRM and K0UO like this.

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