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  • Volt meter wiring

    The voltmeter in my wagon reads .75 - 1.0 volts lower than when I put my meter on the alternator directly. I have tried a number of things to resolve this with little success. Today, I decided to run a wire directly from the alternator to the voltmeter. Now I know if I wire it directly it will eventually drain the battery if it sits for too long. So I decided to use a relay to manage the drain.

    So my question is:

    Will I see less voltage drop through a standard relay (SPDT) if I use the relay to connect the ground instead of the lead from the alternator?
    Last edited by cstmwgn; June 28, 2019, 03:51 PM.

  • #2
    I don't think it will matter. Why not wire it directly with a switch to control the circuit?
    Last edited by oletrux4evr; June 28, 2019, 07:51 PM.
    Ed, Mary, & 'Earl'
    HRPT LongHaulers, 08, 09, 10, 11, 12, 13, 15, 16, 17, 18, 19.


    Inside every old person is a young person wondering, "what the hell happened?"

    The man at the top of the mountain didn't fall there. -Vince Lombardi

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    • #3
      Last two battery drain issues we have had led straight to replacing the alternator. Went from the Jimmy draining over night, to the tracker draining if you dont start if for 3 days. Might be starting to see a constant drain leaking thru some contact with the block.

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      • #4
        what is it you are trying to measure, with the volt meter? The voltage at the alternator, which doesn't really matter, or the voltage in the battery, which does matter?

        My fabulous web page

        "If it don't go, chrome it!" --Stroker McGurk

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        • #5
          Originally posted by squirrel View Post
          what is it you are trying to measure, with the volt meter? The voltage at the alternator, which doesn't really matter, or the voltage in the battery, which does matter?
          Jim

          I will have to check it again but I am pretty sure that it was within .1 - .2 volts difference between the alternator and the battery. The alternator is closer / easier to wire directly as the battery is under the car in the back. What are your thoughts about voltage drop across a relay (SPDT)? Would i see less change if I used the relay to connect the ground wire instead ?

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          • #6
            I'm still mystified by what you're trying to do....maybe explain it a bit more?

            There is voltage drop in the charging system, we generally don't worry about it unless it's so severe that that battery does not get recharged, or there is trouble starting the engine, or lights are dim, etc.

            It's common practice to measure the voltage of the switched power (the "ignition" circuit of the ignition switch).

            Is there some reason you don't want to do this?

            If there is, then you might want to use a relay to connect the volt meter to the battery, and have the "ignition" circuit engergize the relay. You'll still get some voltage drop between the meter and the battery, but it will probably be insignificant if the wire is decent sized for this minuscule load (at least 18 gauge?) and the gauge draws little current. The voltage drop on any wire is proportional to the length of the wire and the current through the wire, and inversely proportional to the area of the wire. The resistance of the relay contacts probably depends on the quality of the relay, if you get one from an automotive parts supplier it will not be as good as an expensive name brand relay from an electronics supplier (the one from the electronics place should also have a data sheet which will tell you the maximum resistance after X number of cycles, etc)

            My fabulous web page

            "If it don't go, chrome it!" --Stroker McGurk

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            • #7
              Just an after thought.....heavier wire might stop the voltage drop.........something else to consider.
              Ed, Mary, & 'Earl'
              HRPT LongHaulers, 08, 09, 10, 11, 12, 13, 15, 16, 17, 18, 19.


              Inside every old person is a young person wondering, "what the hell happened?"

              The man at the top of the mountain didn't fall there. -Vince Lombardi

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              • #8
                heavier wire will reduce the voltage drop. But it will always be there. It's just the nature of electricity.

                My fabulous web page

                "If it don't go, chrome it!" --Stroker McGurk

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                • #9
                  Originally posted by squirrel View Post
                  There is voltage drop in the charging system, we generally don't worry about it unless it's so severe that that battery does not get recharged, or there is trouble starting the engine, or lights are dim, etc.

                  It's common practice to measure the voltage of the switched power (the "ignition" circuit of the ignition switch).

                  Is there some reason you don't want to do this?

                  If there is, then you might want to use a relay to connect the volt meter to the battery, and have the "ignition" circuit engergize the relay. You'll still get some voltage drop between the meter and the battery, but it will probably be insignificant if the wire is decent sized for this minuscule load (at least 18 gauge?) and the gauge draws little current. The voltage drop on any wire is proportional to the length of the wire and the current through the wire, and inversely proportional to the area of the wire. The resistance of the relay contacts probably depends on the quality of the relay, if you get one from an automotive parts supplier it will not be as good as an expensive name brand relay from an electronics supplier (the one from the electronics place should also have a data sheet which will tell you the maximum resistance after X number of cycles, etc)
                  Jim
                  The car keeps the battery well charged and has no issue starting or running all the accessories. I has a 200amp 3G alternator on it which seems more than adequate to handle the two amps, halogen headlights, twin spal fans, A/C and MSD.

                  What it does do is it appears to read low charge on the voltmeter - often less than 12 volts. When I check the alternator output - it is putting out well above what the gauge says. I have changed the gauge thinking it could be the culprit but it didn't solve the issue. I am not all that smart when it comes to wiring and electronics so I sometimes do foolish things.

                  I hate to keep beating a dead horse but would the voltage drop be less at the gauge if the relay was put on the ground side?
                  Wire from ignition switch to energize the relay when the relay is energized it would connect the gauge ground wire to ground.
                  Would this reduce the voltage drop at the gauge and therefore reduce the need to use a higher quality relay?

                  Thanks for your input.

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                  • #10
                    Originally posted by oletrux4evr View Post
                    Just an after thought.....heavier wire might stop the voltage drop.........something else to consider.
                    Morning Ed.

                    Yeah, it was my hope that since it was only running the voltmeter that it would be OK to run an 18 gauge wire. My guess is it is around 5 feet coming from the alternator vs somewhere around 13 feet coming from the battery.

                    This probably another one of my "fools errand" but it seems to bother me even though it hasn't caused any significant issue over the years.

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                    • #11
                      Well, before you permanently wire it, run your circuits with 16 gauge and see how that compares to 18. You can hide the wiring afterwards. Jim will have the right answer pretty soon, but it's pretty early in AZ.
                      Ed, Mary, & 'Earl'
                      HRPT LongHaulers, 08, 09, 10, 11, 12, 13, 15, 16, 17, 18, 19.


                      Inside every old person is a young person wondering, "what the hell happened?"

                      The man at the top of the mountain didn't fall there. -Vince Lombardi

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                      • #12
                        sometimes the lights dim when a big piece of machinery starts up, you know, if electricity is electricity. I would think a 200 amp alternator would do the same thing. I just thought the only gauge you want to hook up to an alternator is a ampmeter to show when its charging.

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                        • #13
                          The volt meter is indicating what the voltage is, where it is being measured. If the car is working fine, and it bugs you that the voltage reading is not what you think it should be, then disconnect the damn meter and get on with your life.

                          If you want to fully understand what is going on, and see if there is actually something that needs to be done, then we can probably help you. But it still doesn't matter what the voltage at the alternator is, what matters is the voltage at the battery (so you can be sure it's getting charged).

                          As for voltage drop on the positive and negative side...electricity flows in a loop, it's called a "circuit" for a reason. The voltage drop in the negative side of the circuit might be less, if you are using a larger conductor (such as the body, frame, engine block, etc) as part of the circuit.

                          I don't see that it has anything to do with how you wire your volt meter, though.
                          Last edited by squirrel; July 1, 2019, 06:16 AM.
                          My fabulous web page

                          "If it don't go, chrome it!" --Stroker McGurk

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