Measuring wattage off panels with volt meter and MC4 connection

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ClearHorizon
ClearHorizon Registered Users Posts: 4
in Solar Beginners Corner #1
I'm not a total newbie, but I just got some 20V panels with MC4 connectors that go to a MorningStar 45 controller which does not show the wattage coming off the panels. This set of panels is running in addition to the original array of six 75-watt panels and a Trace controller. Due to time constraints and daylight disappearing, I wasn't able to put the volt meter on the bare wires coming off the new panels before connecting them to the MorningStar controller. The old array is charging the batteries, too, so it's not evident that the new panels are putting off what they are supposed to. They should be, there doesn't seem to be a problem, but I'd like to check.

Now that it's all connected up and running, is there a place on the MC4 connectors that I can put the volt meter to check the panels and what they are doing? I know eventually I could get the remote meter for the MorningStar, but don't have it yet. Thanks.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Measuring wattage off panels with volt meter and MC4 connection

    Welcome to the forum.

    MC4 connectors are "weather tight" and not designed for intrusion. The Voltage on these wires will be the same as any other parallel connection, so if all panels are parallel to the charge controller's input that is what the Voltage will be on any of them.

    Of course Voltage isn't the whole story for power. If you had a clamp-on DC Ammeter you can measure the current coming from each panel. That would give you a better indication of what power is coming from each panel. Likewise checking the output current from each charge controller in this manner will tell you how much each array is contributing to charging.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #3
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    Re: Measuring wattage off panels with volt meter and MC4 connection

    Assuming the charge controller is working correctly and you can use the display or computer interface to access the controller's readings (or just a DMM on the PV input terminals and a DC Current Clamp Meter)...

    By the way, is this a MPPT or PWM TS 45 MorningStar charge controller. And can you tell us the panel Vmp/Imp for each string, how many panels you have in series for each string, and the battery bank voltage.

    First, have a discharged battery bank (below ~80% state of charge) and/or turn on a heavy load (electric heater, etc.) to dump excess power.

    Disconnect the strings. And one string at a time, connect it to the controller. Measure the PV Array voltage and PV Array current for each string.

    Now compare them to each other, and to the panel specification for each string.

    Note that the readings and mix/match issues will be somewhat different depending on if you have a MPPT or PWM charge controller.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • ClearHorizon
    ClearHorizon Registered Users Posts: 4
    #4
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    Re: Measuring wattage off panels with volt meter and MC4 connection

    Caribootcoot, thanks for the fast reply! A DC ammeter, great idea. I should probably have one around for other things as well. Thanks!

    BB, it's an MPPT controller, and all I want to check first is that the new panels are putting out what they are supposed to in full sun. The MorningStar controller is in agreement with the original Trace controller as far as battery condition and SOC.

    The Trace shows the wattage from the panels and the fluctuations, which I like to see happen throughout the day and the seasons. The remote meter is in my future, but just adding these two new 265W, 20V panels and a controller cost a pretty penny as it is!
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #5
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    Re: Measuring wattage off panels with volt meter and MC4 connection

    If you have total current output display somewhere, either with a battery monitor or in the CC, you can simply cover one panel t q time withq blanket and have someone watch to see if the charge current (and or voltage) drops. I do this regularly to check to make sure I don't have a bad connection somewhere. I drape each panel, and the output drops exactly the output of that panel. If it doens't, it means I've got trouble.



    Tony
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  • ClearHorizon
    ClearHorizon Registered Users Posts: 4
    #6
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    Re: Measuring wattage off panels with volt meter and MC4 connection

    Icarus, thanks for that idea. The MorningStar doesn't show voltage, it only has lights, and combinations of lights, and even those are probably not that sensitive. But if the original Trace controller responds to the new panels via the batteries, I can keep an eye on that and see if it registers small changes.

    BB, now that I think of it, I'll have to find out which uses more power 24/7, the PC connection (which could probably be disconnected when not needed) or the remote meter. The controllers are 60 feet from the house, so it's another project to run another line.

    Do you have any fog issues in the Bay Area?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #7
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    Re: Measuring wattage off panels with volt meter and MC4 connection

    I am near Highway 92 and 101 (San Mateo)--It is, on average, pretty sunny weather here most of the time.

    I spent the first 30 years of life in Pacifica--4 days of fog, 2 days of sun... Went to SF State U, and it would be foggy/overcast the entire "summer".

    You wan the batteries discharged/under load so the controller(s) will output their maximum current to the battery (MPPT mode, or PWM at 100%) once the batteries are ~80-90% full, the batteries will be at "absorb" voltage and slowly dialing back on the current. Perfectly normal, but of course, very difficult to figure out if you have a working solar array or not.

    In general, if the measured wattage of your array around noon is approaching 77% of rated power--You are doing OK. If your measured power is 50% or less on a sunny day, you are probably having problems.

    With multiple, same model panels, you are doing A/B comparrisons--So if something is "wrong", it is easy to see when compared with the other panels.

    Again, knowing the Vmp/Imp of the panels, and how they are connected in series/parallel to the charge controller is important too. A 24 system with "24 volt" panels is not always a workable system.

    A true "24 volt panel" for battery charging should have Vmp in the 35-39 volt range (STC--standard test conditions). Many "24 volt panels" advertised as such, have Vmp in the range of 30 VDC... That voltage falls by 3-5 volts when the panel "heats up" in full sun--And you need 29+ volts to charge a 24 volt lead acid battery bank.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #8
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    Re: Measuring wattage off panels with volt meter and MC4 connection
    BB. wrote: »
    Assuming the charge controller is working correctly and you can use the display or computer interface to access the controller's readings (or just a DMM on the PV input terminals and a DC Current Clamp Meter)...


    -Bill

    Bill I think your link has changed, is this the same meter?
    http://www.sears.com/craftsman-digital-clamp-on-ammeter/p-03482369000P
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #9
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    Re: Measuring wattage off panels with volt meter and MC4 connection

    Yes that is it--for some reason the "old link" still works for me too... Perhaps something is cached in my system/browser.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • PNjunction
    PNjunction Solar Expert Posts: 762 ✭✭✭
    #10
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    Re: Measuring wattage off panels with volt meter and MC4 connection

    Suggestion about that meter - I use a similarly priced unit (probably the same internally), and found the best accuracy was obtained by using non-rechargeable lithium AA's. This got the accuracy much closer to my Fluke 87V. When the clamp meter was run from alkaline, accuracy was off a bit and going downhill the longer I used it.

    I can only claim that my unit (Radio Shack #22-172) doesn't seem to mind AA lithiums. Not sure about the Sears.
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  • ClearHorizon
    ClearHorizon Registered Users Posts: 4
    #11
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    Re: Measuring wattage off panels with volt meter and MC4 connection
    PNjunction wrote: »
    Suggestion about that meter - I use a similarly priced unit (probably the same internally), and found the best accuracy was obtained by using non-rechargeable lithium AA's.

    Thanks, PNjunction, that's good to know. It's a small system and the more even small things that are on it 24/7 take it down more than you'd think.


    BB, yeah, Pacifica. I am pretty far south of you, but went on a tour of PG&E roof installations in Pacifica about 6-7 (?) years ago. For $20,000 they got these people to be grid tied, even talked a few of them into going to electric heat, and I thought at the time, this is the biggest ripoff imaginable. The panels up where they can't get at them and fog in the summer. I don't know how they feel about it now, have you ever heard? Even just a smattering of pine needles on my panels and they drop quite a bit, but they are on poles on the ground so I can clean them. Another reason to know just how the panels fluctuate during the day.

    The two 20V panels (not 24) are in serial, so they are up to 40V with the MorningStar MPPT controlling them to suit the 24V batteries. It does my soul good to see 75% of the panels coming through. This is my first dual controller setup, I want to watch how it all interacts.
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