Shunt reading and selection

zeuspaul
zeuspaul Solar Expert Posts: 59 ✭✭✭✭
in Advanced Solar Electric Technical Forum #1
I want to size a shunt and read it with a multimeter. The system will be 1350 watt PV nominal, 225 amp hr 24 volt battery and a 2000 watt / 4000 watt surge inverter.

A common shunt is the 500 amp 50 mv. Do I divide 500 by 50 to get 10 amps per mv. If I read 2 mv on the voltmeter does that represent 20 amps?

A ten to one ratio seems to make sense because the math is easier. However the max current rating on an Exeltech 2000 watt inverter is 224 amps at 21 volts. Would it be better to get a shunt more closely matching the inverter rating?

Does a 300 amp 100 mv shunt represent 3 amps per mv? If the volt reading is 5 mv does that represent 15 amps? The math is less convenient. However is the accuracy better with 3 amps per mv vs 10 amps per mv? Are there other considerations?

Zeuspaul
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #2
    Re: Shunt reading and selection

    You have the math correct.

    Measuring millivolts (and smaller) with a DMM can be a pain at those low of voltages--so having higher voltage readings can improve accuracy (or you might need to get a meter than can measure 100's of micro volts).

    However there are power losses and surge currents to worry about too with higher voltage shunts...
    • 0.100 volts * 300 amps = 30 watts of waste heat from shunt
    Also, if this is a 12 volt system, 0.100 volt shunt is just more voltage drop causing you problems too.

    -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 ✭✭✭✭
    #3
    Re: Shunt reading and selection

    DVM's, at best, generally have no smaller then a 200 mVdc scale. This is not really sensitive enough to read the few mVdc from a 500 A / 50 mV shunt that yields only 0.1 mVdc per amp.

    You can build a low DC offset / low drift / low noise differential op amp to amplify the small voltage from the shunt if you you have the expertise and inclination. This amplification is what is in all battery monitors that use such a shunt.

    Or buy a Trimetric battery monitor for about $150.
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  • zeuspaul
    zeuspaul Solar Expert Posts: 59 ✭✭✭✭
    #4
    Re: Shunt reading and selection

    Thanks for the responses.

    The shunt selection seems to be a choice of one which makes the decision easy. There is little support for any shunt other than 500 amp 50 mv in the common battery monitors. Reading with a voltmeter is a stop gap measure. Support for a battery monitor is important.

    A clamp-on meter as an interim measure would probably give more accurate results than reading the shunt with a voltmeter.

    Thanks

    Zeuspaul
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #5
    Re: Shunt reading and selection

    I have never understood why the 500a 50mv shunt is so widely used whan a 200a 200mv shunt is much more easy to get an accurate reading from with a average digital multimeter.
    200a on 12v = 2400w.. who uses above 2400w on 12v?/ and if you do you shouldnt in any solar situation.
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