Accurate Current measurements for SOC.

bmet
bmet Solar Expert Posts: 630 ✭✭
in General Solar Power Topics #1
I see some posts that refer to the subject of shunts, but are they necessary in a small power pack(10A pwm controller and an 85AH battery)? My interconnects are all #10AWG, insulated solid copper, the largest my controller's terminal will accept.

I have a mid-range Craftsman DMM, rated to 20A.

Placing the meter in series with the load gives real-time Current draw, but would the skimpy leads of the meter be a problem? DMMs with a separate connection for higher Current, use a precision resister. Would that be helpful for determining SOC?

Thank you.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #2
    Re: Accurate Current measurements for SOC.

    Your Craftsman DMM will have too much series resistance by itself, not even including the leads.

    You don't want to add more then 50 to 100 mV of voltage drop due to the current measurement.

    Most DMM do not have sensitive enough base meters. Typically no better then a 200 mVdc minimum voltage scale.

    100 mVdc developed across a shunt with 10 amps would be a 10 milli-ohm resistor.

    Shunts typically come in 100 amp (1 milli-ohm) and 500 amps ( 0.1 milli-ohm).
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #3
    Re: Accurate Current measurements for SOC.

    If I may ask for a clarification, are you hinting that the Current measurements may be inaccurate because it's in series with the Load? I'd expect this with a $3 Harbor Freight meter, but a $60 Craftsman? Now that I think about it, my meter can use an optional Current clamp, but I didn't purchase it :(
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #4
    Re: Accurate Current measurements for SOC.

    The issue is that your meter will add a significant voltage drop in your wiring.

    It will work "OK" if you use it in series connections but in parallel connections it can change the current flow (say you wanted to measure the current flow between two paralleled batteries in your bank).

    The typical current clamp will usually be for AC current only (current clamp based on a transformer) and would not work for DC. You need an "active" current clamp that uses a Hall Effect transistor to measure the magnetic fields and report them back to your meter. (like this integrated AC/DC clamp current meter).

    For a small system, you might want to look at an Amp*Hour / Watt*Hour meter for DC battery systems. Very handy to measure the amount of energy over time that is leaving (or entering) your battery bank/loads.

    The above meter only increments (Amp*Hour increases only). A battery monitor actually counts up and counts down based on the direction of current flow--Sort of like estimating the amount of fuel in your gas tank by logging fuel flow into and out of the tank through the fuel line.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #5
    Re: Accurate Current measurements for SOC.

    Sears 'Craftsmans' lableled version of MA220.

    http://www.sears.com/shc/s/p_10153_12605_03482369000P
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #6
    Re: Accurate Current measurements for SOC.

    Thank you RCinFLA, Thank you B.B.
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