Mobile Radio Install DC Power Myths

Discussion in 'Mobile Radio Systems' started by KF5LJW, Sep 12, 2019.

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  1. KX4O

    KX4O Ham Member QRZ Page

    KF5LJW, thank you for the advice. I've long been hesitant to install my HF mobile rig per the advice found in transceiver manuals and other resources... something just never quite made sense. I wasn't in a rush so just put off the install until I gain confidence I won't set my wheels ablaze.

    What about those of us who have the BMS system sensor on the positive battery lead? Should we bypass the BMS sensor and connect straight to the battery terminal or to the "car side" of the sensor?

    As WB2LBV implied...
    ...this isn't our father's automotive power system anymore.
    Last edited: Sep 17, 2019
  2. KF5LJW

    KF5LJW Ham Member QRZ Page

    Good question. If you find the sensor on the Positive, then connect to the LOAD or Car Side in your terms.

    What I can tell your for sure is in GM, Ford, and Chrysler White Papers and Service Documents say specifically the Battery Positive Term Post. With that said I would assume Ford, GM, and Chrysler use a Sensor on the Negative Lead. At least th eones I have looked at do, but that is not all models.

    Here is the ACID TEST. Pop the hood and look at the battery closely at the Battery Positive Term Post. If you see an EMPTY Terminal or Empty Fuse Position, that is where the manufacture intended you to terminate. I also bet if you find a spare terminal or fuse, the sensor is on the Negative Bonding Jumper.

    On the flip side if you see a sensor on the Positive and no Spare Terminals or Fuses on the Battery Term Post, follow the red wire to the Battery Distribution Fuse Block, and there you should find a spare Terminal or Fuse for your radio as designed.

    One way or the other, the auto manufacture provided a means. Use it.

    Still confused, don't ask an ham radio operator or radio manufacture. Take your car down to the dealership and do something unthinkable. ASK. They will tell you and it will not VOID YOUR WARRANTY. Ask a ham or radio manufacture and you are likely to void your warranty if you choose poorly.
    KX4O likes this.
  3. KI5GKD

    KI5GKD Ham Member QRZ Page

    Well, I wondered about that "extra" fuse when I installed my ICOM IC-2300H last weekend. I remember thinking that it made no sense to fuse the negative line but figured THEY knew what they were doing so I left it be. Tomorrow the negative lead gets moved and fuse taken out of the picture... :mad:
  4. KF5LJW

    KF5LJW Ham Member QRZ Page

    FWIW if you wanted, you could leave the Negative under the hood. Best place to terminate in the engine compartment if used is the same place the battery chassis cable terminates too usually found on the wheel well. Easy to find. Essentially your on Battery Negative Term Post or as close as you can get safely. If anything were to happen to the Battery Cable, you radio would be disabled along with the car. No vehicle current can flow through so you still have a dedicated circuit.
  5. KI5GKD

    KI5GKD Ham Member QRZ Page

    OK, didn't get a chance to mess with it today as the wife had relatives from out of town so I had to socialize. I'll try to eliminate the negative fuse tomorrow. I think I clamped the negative lead under a bolt that was attaching a brace to the firewall so might leave it there and just eliminate fuse.
  6. KF5LJW

    KF5LJW Ham Member QRZ Page

    That should be just fine.
  7. KI5GKD

    KI5GKD Ham Member QRZ Page

    I lost the fuse and moved it from the passenger's side to the corresponding brace on the driver's side.
  8. KF5LJW

    KF5LJW Ham Member QRZ Page

    Got an interesting update that confirmed my suspicions on radio manufactures. Doing work with a large utility installing one of the largest P25 Networks in the country across 11 states. Involves over 2100 mobile radios. So I quizzed the various telecom departments how they terminate the negative to the radio. 3 of the 23 shops said the Battery Return Post with a Fuse. So I asked how they come up with that, answer was radio manufacture instructions.

    Being at the parent company, we just happen to have a factory rep as a contract employee aiding engineering with the design and deployment of the network. So I got with him and asked to be put in touch the mobile technical staff to discuss power wiring. Of course my direct question is what is the logic of terminating directly to the Battery Term Post with a Fuse?

    The answer did not surprise me at all, same reason I did it 40 years ago. To minimize voltage drop in early uni-body chassis design. So I said OK, why the fuse? I got an honest answer, in the event the vehicle Battery Return Circuit failed to keep stater current from flowing through the radio. I laughed and had one card to play.

    So now I played my Ace and asked what prevents normal load current from flowing through the radio when the vehicle return circuit is normal and operational. I received a very STUPID ANSWER. They actually said "Current Takes The Path of Least of Resistance". So there thinking is; as long as the auto battery cables are OK no vehicle load current would flow through the radio. That triggered a Come To Jesus Meeting and confess your sins.

    How many of you have heard the term: Current Takes The Path of Least Resistance? I certainly have more times than I can count over 40 years. How many of you believe that BS? Shame on you if you believe that. You failed week 1, day 4, Parallel Circuit Law. Fact is: Current takes every path available to it.

    So what I had those morons do is perform a simple test to prove to them that in fact vehicle current flows through the radio wiring under normal operating conditions. I had them take their own test vehicle and had than take current measurements in both Positive and Negative of the radio wiring.

    Test 1. Measure current in both polarities with vehicle turned off with keys removed, radio in RC
    Result current was equal in both polarities measuring .8-amps

    Test 2. Repeat above test except this time with engine running and all accessories turned ON
    Result: Positive polarity current = .8-amps and Negative polarity = 2.9-amps

    Test 3. Repeat as above except while craning engine

    Result: Positive = .6 amps, Negative = 13.5 amps

    Surprise surprise, surprise, normal vehicle load current flows through radio under normal operating conditions. This is what is known as a real Ground Loop, the kind that gives ground loops a bad name. So I had them make two modifications to their test vehicle. Remove Negative of the radio from the Battery Return Post and move it to the other end of the battery cable on the chassis. Once completed, repeat test.

    Well you can guess what happened. Current remained equal in both polarities like it is suppose to be at .8--amps in standby RX. To their surprise, not mine, they noticed RX performance improved, the noise floor dropped 10 dB. All I could say is DUH, you got rid of all the vehicle current in the radio nosing things up.

    So for you guys who terminated the radio negative directly the Battery Term Post, try the test yourself. When done ask yourself why you terminated to the Battery Term Post, and do you want all that nasty ground loop current flowing through your radio?

    As a point of interest, the manufacture also measured current in the Battery Bonding Jumper going to the chassis. What they found amazed them. 7% of all vehicle current passed through the radio. I kind of laughed at them, and said; nothing amazing at all, just Ohm's and Parallel Circuit Laws at work.

    W1TRY, KD4MOJ, WC5P and 1 other person like this.
  9. N4MU

    N4MU Ham Member QRZ Page

    VERY interesting! Thanks for the post...again. I have just finally completed my mobile install (FT-857D in the rear seat "cubbie" and Ford F-350 diesel). Got the TX drop down to 0.6 volts so am happy. Yes, noise dropped by negative lead going to seat bolt, etc. Everything tunes/sounds great...except the "injector shuttle" noise (per Alan's - K0BG accessment that's what it is and I agree) is still very much there (S8). If I could only get rid of that (even half of it!) it would be a perfect world! Well, almost....
    KX4O likes this.
  10. KF5LJW

    KF5LJW Ham Member QRZ Page

    .6 volts is a bit high IMO as that equates to about 5%. Might want to consider using a larger gauge wire. Ideally you want voltage drop at 3% or less which is roughly 0.4 volts or less.

    Great! That is what everyone should experience by terminating the Neg in the correct location. Additionally you should have less voltage loss. It is Win-Win vs Lose-Lose terminating to battery.
    Last edited: Sep 24, 2019

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