shunt selection

rama
rama Solar Expert Posts: 36 ✭
in Solar Beginners Corner #1
hi all,
Is there a general guideline on how to go about choosing a right-sized shunt? I know that they're usually rated in terms of mv and amps, but how do I choose?

There is no particular project in reference - it's just something I've always been wondering about.

Thanks & appreciate for any input
-erick
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,654 admin
    #2
    Re: shunt selection

    First is the maximum current (100 amps, 500 amps, etc.).

    Second is to pick the shunt that matches the calibration of the meter/battery monitor (100 amps @ 100 mV; 500 amp @ 50 mV; etc.).

    Note that the ratio of current to mV can be played with to the get "correct value"... For example 500amp @ 50 mV and 1,000 amp @ 100 mV have the same ratio (calibration) as far as the meter is concerned (10 amps = 1 mV).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: shunt selection

    Yes; you follow the manufacturer's instructions.
    In other words if you buy a Trimetric and it says you need a 500 Amp shunt for your application that's what you use.
    That may sound horribly simplistic, but it works.
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #4
    Re: shunt selection

    Dont use a shunt that is to higher rating than what the load will be.
    example dont use a 500 a shunt if your load is only about 8a as the meter reading will be to inaccurate
    conversely if your load is 48s dont use a 50 a shunt , use a 100a one.
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  • rama
    rama Solar Expert Posts: 36 ✭
    #5
    Re: shunt selection

    Hi - thanks for all your input.

    At what size would a shunt be considered TOO HIGH a rating for, say, a 50Amp MPPT controller? The manual states that a 500Amp/50mv should be used, but given what I just read here I should probably just go with a 100Amp one?

    Thanks.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: shunt selection

    :confused: What charge controller requires a shunt? That isn't a typical application in my experience.
    Maybe I've done too much today and am just too tired to understand. :blush:
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,654 admin
    #7
    Re: shunt selection

    Start with your "peak" current flow (either into or out of the battery bank).

    If you draw a little power (say HAM radio) 24x7, then your charging current may be the peak current.

    If you have a well pump on an Inverter, then that will probably be your peak current.

    The idea is that the shunt/meter combination can measure (and safely pass) your maximum current flow.

    Also, you need to review your battery monitor's manual. Some may be programmable for a whole bunch of shunts, others may require one specific shunt.

    What battery monitor/meter are you looking to attach to the shunt?

    Besides the solar charge controller, what other "high current" loads/charging sources will be on the bank?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #8
    Re: shunt selection

    Using a 500a shunt between either the panels or battery/ies is insane, use a 100a one far more accurate reading.

    cariboocoot I will tell this but after I do I know EVERYONE will give me all the reasons why its wrong to do so, and in the reasons all will contradict themselves in their reasons.

    I use a 50 a shunt before and after the PWM charge controller and 100a one between the 1000w inverter and the batteries. WHY ??? it gives me the readings of amps from device to device and it acts as a fuse if a major electrical problem occurs.
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  • rama
    rama Solar Expert Posts: 36 ✭
    #9
    Re: shunt selection
    Cariboocoot wrote: »
    :confused: What charge controller requires a shunt? That isn't a typical application in my experience.
    Maybe I've done too much today and am just too tired to understand. :blush:

    lol... ok let me explain myself. We got this charge controller here that "suggests" the use of a shunt for the battery so the controller can measure current in/out of the battery. I'm just trying to figure out what is the proper current rating I should choose in order to size for the right shunt. The controller has a 50A MAX charging current, but suppose the battery's hooked up to, say, a 2KW inverter operating at 24V, then there's the max current of about 83AMP. I should take the 83amp as reference?
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  • rama
    rama Solar Expert Posts: 36 ✭
    #10
    Re: shunt selection

    ok - sorry never mind I see my questions have already been answered. You guys are super fast... Thanks!!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,654 admin
    #11
    Re: shunt selection

    To get a little more specific:
    • 2,000 watt inverter * 1/21 volt minimum * 1/0.85 inverter eff = 112 amps maximum
    Now, will you run your inverter at 2,000 watts and 21 volts (battery cutoff voltage)--probably not very often (if ever).

    And--it does depend on where you install the shunts... For a Battery Monitor, they are usually installed between the negative battery post(s) and the rest of negative connects so that you "monitor" all of the current going into and out of the battery bank.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • rama
    rama Solar Expert Posts: 36 ✭
    #12
    Re: shunt selection

    thanks BB - your thorough explanation with numbers are always much appreciated..:D
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #13
    Re: shunt selection
    john p wrote: »
    ......
    I use a 50 a shunt before and after the PWM charge controller and 100a one between the 1000w inverter and the batteries. WHY ??? it gives me the readings of amps from device to device and it acts as a fuse if a major electrical problem occurs.

    Shunts are supposed to be able to get quite hot, and I imagine they only "fail" when the solder or brazing lets go. I'd rather use a fuseable link from a car.
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #14
    Re: shunt selection

    Mike if a 100a shunt is between a battery and a short in a device they fail at the laser cuts on the resistor and tend to not melt but "vaporize". ones with no laser cuts are usualy not that accurate.I try never to use them
    A 100a shunt can withstand about 110a indefinately 120a for one hour 130a 30 mins 140a 15 mins 180a about 2 seconds.
    have tested many many and found very consistant results,, far better at predictable failure than most fuses.

    We use then in big quantities at work as combined fuses/shunts., does 2 jobs very well,cheap, easy to replace
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