"Independent" battery power in a car

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

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  1. AG6QR

    AG6QR Premium Subscriber QRZ Page

    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.
    W4POT, AI7PM, W9WQA and 2 others like this.
  2. KN4YRM

    KN4YRM XML Subscriber QRZ Page

    Great analogy! Sorry I didn't explain it that way in the first place. It's exactly like using a battery-powered HT with a magmount in the car, schematic wise.
    AI3V likes this.
  3. W9WQA

    W9WQA Ham Member QRZ Page

    waaayyy tooo much worrryying going on here.

    just run it.....
    lots of wasted words. !!!
    WD5IKX likes this.
  4. KB3WFV

    KB3WFV Premium Subscriber QRZ Page

    Very cool. I get it. Your wanting to isolate the antenna from the house during bad weather by using the car while not killing the car battery. I'll assume that you have the proper charger for the Lifepo battery then. You are good here.

    There is no need to do anything with the car battery at all. In fact,. depending on what kind of car it is, you may cause more headaches by disconnecting a car's battery without providing a backup power source for the onboard computer control modules. Leave the battery in the car alone. I see disconnected/reconnected (without backup) car battery induced problems in the my shop too often.

    Set a fully charged Lifepo battery on the floor board of the car. Connect the radio to the battery, plunk the mag mount on the roof and connect the coax to the radio. Tune it up and have fun. There is no need to connect anything from the lifepo battery to the car or connect anything from the radio to the car. The battery and the radio is the DC circuit. The antenna/car and the radio is the RF circuit.

    There will be no interaction between the antenna and the car battery for two reasons.

    1) The connection between the antenna ground and the car battery negative is not a complete circuit.

    2) The ground connection at the antenna is really an RF Return Path. A mag mount is inductively coupled to the metal body of the car providing an return path for RF signal to the radio. The 12 DC battery will never see RF voltages.

    The car's battery connection to the car body is only a DC ground. Leave the car battery alone! It will never ever see RF voltages. You are basically operating portable. Instead of a picnic table in the middle of a field with a battery, a radio and a hamstick with a some wire for a RF return path or ground plane, you are using a battery, a radio, a hamstick and the metal body of a car for a RF return path or ground plane. Battery to radio, radio to antenna. No battery connection to the car needed and no radio connection to the car needed... No problem here.

    Use fused Anderson power pole connectors to the the lifepo battery and conect the radio when operating (as you say) remote. To operate mobile, install fused power pole connectors to a power source in the car. This way all you ave to do is switch the radio between the car battery and the lifepo battery. Easy!

    Operating mobile, (which is really what you are doing just with an alternate power source) is a lot of fun. It will require a little more work fpr best performance such as bonding doors etc to the body. but that ca be done as your project progresses. Set everything up (radio and antenna wise) as if you were operating 100% mobile. All of the mobile installation and operating theory's apply with your set up. The only difference is that sometimes you will run the radio from an alternate battery power source and do it remotely. I think that is pretty cool!

    Read up on the web site I linked to in my last reple www.k0bg.cpm. It is a wealth of information on mobile operations.

    Ya know, it might even be better to set something up to operate a fixed antenna in the yard remotely. A dipole, a battery, a radio. You could add a solar panel to keep the battery charged. Now, that would be cool too.

    KN4YRM likes this.
  5. KN4YRM

    KN4YRM XML Subscriber QRZ Page

    Currently that car is only set up with vhf/uhf radio, so I wanted to use the opportunity to see if I can get an HF mobile antenna working with the vehicle anyway so when this lockdown is over I'll be able to work "real" HF mobile too. But all the things you mentioned are good ideas, and no reason I really have to use a car long term either if the real goal is just keep lightning out of the shack on overcast days but still get on the air with something. Thought it might also be a cool proof-of-concept for those who live under antenna restrictions, just use your car in the driveway and the neighbors will never know! Though I doubt I'd be the first to do this anyway...
  6. K8ERV

    K8ERV QRZ Member QRZ Page

    I would use a goodery in place of the second baddery.
    Gives my spell checker something to do.

    TOM K8ERV Montrose Colo
  7. WD5IKX

    WD5IKX Ham Member QRZ Page

    I did what you are saying for Field Day this year. During the night I operated from two 91 AH AGM batteries that had been charged with a solar panel system. I did not have to worry about starting the truck when I got ready to leave. I use power pole connectors to swap the power source and everything is grounded and bonded for good mobile operation.


    John WD5IKX
  8. K4NYM

    K4NYM XML Subscriber QRZ Page

    This is EXACTLY what I do. I have two Bioenno 20Ah LeFePO4 batteries. I alternate between them. I do a lot of POTA (Parks-on-the-Air) work and use hamsticks. I have quick connections on all my hamsticks and attach them to a mag mount on the roof of my Camry. My rig is an FT-891. I have each hamstick tuned to the center point of each band and use an MFJ-939Y to even out the SWR on either side of the band. This set up works great with the ability to reach each each state CONUS as well as a healthy amount of DX. If you're mobile/portable operation is similar to mine then you should not have any problems. If you're not using your car battery then your car really has nothing to do with your set up. Now, if you use the car's battery, that is different.
  9. NG1H

    NG1H XML Subscriber QRZ Page

    Have a small automatic charger with a quick disconnect hooked up to your starter battery and just leave the radio hooked up to the car's electrical system. Check the brand and model first to make sure that it is RF quiet. Simple, easy, and now you have a system already in place if you ever need to leave your car for an extended period due to trips. You get home, plug it in, and just leave it hooked up overnight. You won't be using that much power in normal operations and a 1 or 2 amp charger should replace it.
  10. KD1ELK

    KD1ELK XML Subscriber QRZ Page

    I put a 100 watt solar panel on my Tacomas roof rack and hid a solar controller and two 35ah batteries behind my rear seats. From there I ran several power plug mounts inside the cab and into the bed under my shell. I run my radios on it as well as an off road fridge and whatever else I need.

    1. Having batteries separate from the vehicles battery is peace of mind. Unlike what another poster said, running 100 watts (20+ amps) for hours with high duty cycle is terrible for a starter type battery. Even if it didn't leave you stranded, it will shorten its life a lot.
    2. You've already been given your answer on RF vs DC ground, but I will just confirm I have never had a voltage issue.

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