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Thread: Shunt Polarity?

              
   
   
  1. #1
    Junior Member calvin2's Avatar
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    Shunt Polarity?

    I bought this ammeter gauge kit from ebay the other day. The gauge kit does not include instructions and is made by VDO and includes a shunt. Upon closer inspection, the shunt has a "+" label on one of its terminals? I was planning on hooking this shunt up from one post to motor "+" and the other post to controller. Anybody have a clue on whether the "+" post goes to motor or contorller? Perhaps this polarity only refers to the wires that go to the gauge?
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    EVangelist electriKAT's Avatar
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    A shunt is just a conductor of small, but known, resistance. So polarity is not relevant. I think you are right that the marking is for the wires that go to the gauge. But you might as well hook it up so the markings are correct. In your case, attach the positive terminal to the controller.

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    I'm sure you're both right, but just making err doubly sure.
    In most cases, if not all, simple controllers will pull the motor neg low to batt neg and so you would put the shunt pos to motor neg and the shunt neg to controller out.
    However, if you are using the controller as a junction, i.e. to pass the batt pos on to the motor pos then yes you can put the shunt pos toward this junction and the shunt neg to motor pos.

    No harm done if wrong way round, your meter will just read backwards!

    Rob

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    Empulse R #24 frodus's Avatar
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    why do you care about motor current?

    Lots of the EV's I've seen measure battery current.... what do all you guys use?

  5. #5
    GUFF
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    I had both battery current (paktrakr) and motor current (shunt and meter) in my first conversion. I will most likely get battery current only (until I can make my own display unit) from the Curtis controller/spy glass on my second conversion.

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    Junior Member calvin2's Avatar
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    Quote Originally Posted by frodus View Post
    why do you care about motor current?

    Lots of the EV's I've seen measure battery current.... what do all you guys use?
    Good point. I would think battery current measurement is more pertinent. In this case, battery "-" terminal to shunt terminal "+" and other shunt terminal to controller?

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    Quote Originally Posted by calvin2 View Post
    Good point. I would think battery current measurement is more pertinent. In this case, battery "-" terminal to shunt terminal "+" and other shunt terminal to controller?
    Other way round.
    If you're putting the shunt in the batt neg lead, connect the shunt pos to controller.
    If you're putting the shunt in the batt pos lead, connect the shunt pos to batt pos.

    Rob

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    Empulse R #24 frodus's Avatar
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    it follows the flow of current from the positive lead of the battery pack to negative lead of the battery pack. As you put things in series, everything goes from + to -, then + to -, repeat.

    Its only going to make the gauge display negative current



    But yeah, I think you'd want to watch battery current more closely. Motor current can be many times the battery current (i.e. they're not always equal, in fact, they're seldom, if ever equal). You could have full amps on motor side and only 100A on the battery side. You just want to make sure you're not going over the battery discharge limits.

  9. #9
    Junior Member calvin2's Avatar
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    I have a KD72401 controller (400A max). Max discharge of my AGM is 450A. So I should be within the max discharge limit of my AGM.

    Thanks for the clarificaton on the gauge setup.

  10. #10
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    Very timely thread ... I was wondering the same thing!

    Thanks!

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