Car Ammeter

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  • barry wilkins
    Senior Member
    • Aug 2009
    • 172

    #1

    Car Ammeter

    I did this test on my previous car to check the alternator was charging the battery properly.
    The earth cable from the negative terminal to earthing point on the vehicle has a very small resistance. So I built a simple millivolt meter and connected from the said battery terminal and earth point and measured the voltage drop ,then I connected a DC ammeter in series with the earth cable and measured the actual current drawn.
    Now I had the measurements I calculated a factor ,multiplying the mV by 0.921 which gave me the current .
    Examples
    Side lights - 3.8mV x 0.921 = 3.5A discharge.
    Head lights -12.6mV x 0.921 = 11.6A discharge
    With the engine running after starting gave me +7 to 4.5 mv ,so I could tell the alternator was charging the battery.

    Barry Wilkins
  • tom66
    EVs Rule
    • Apr 2011
    • 32560
    • UK

    #2
    Re: Car Ammeter

    Useful info. But isn't it enough to see the battery voltage increase? A charging lead acid will show around 14.4V, whereas a battery that has been idle will drop down to around 13.5V reasonably quickly.
    Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
    For service manual, schematic, boardview (board view), datasheet, cad - use our search.

    Comment

    • stj
      Great Sage 齊天大聖
      • Dec 2009
      • 30923
      • Albion

      #3
      Re: Car Ammeter

      you can get a digital ammeter with a 200a shunt on ebay for under $10

      Comment

      • redwire
        Badcaps Legend
        • Dec 2010
        • 3900
        • Canada

        #4
        Re: Car Ammeter

        It's an interesting experiment.
        Copper is not so great for a (shunt) resistor. The tempco of copper is about +2% per 5°C and you have to make a Kelvin connection with your multimeter.
        If your probes are not matched- touching different metals like lead, zinc, or copper then thermocouple effects add mV too.

        You have a 1.09 milliohm resistor, all for free. I looked at #4AWG wire which is about 0.25 milliohms per foot, so your crimp connections are looking dirty....

        Comment

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