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Mixing modules with slightly different Imp and Vmp?

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  • Mixing modules with slightly different Imp and Vmp?

    I had 24 flawlessly working 12V 100W modules wired in six series strings for 48V using a Outback inverter and MX60 MPPT charge controller. Three of the modules were recently shattered when a tall pine tree fell on them. The charge controller is currently remaining in standby mode due to insufficient voltage being produced by the array with those 3 modules down.

    I'd like to replace the 3 damaged modules but unfortunately Sunwize, the manufacturer, no longer makes the modules I have. I've located somewhat comparable 12V 100W modules, however their Imp and Vmp vary slightly. My current modules have an Imp of 4.35A and Vmp of 23V while the new replacement modules I am looking at have a rating of 6.54A and 17.1V respectively (there are a few other possible ones but they all seem to range by 1-2A and 4-6V). Will this cause any issues? Thanks!

  • #2
    Hi Ahisma - Welcome to Solar Panel Talk!

    Mike or Sunking should be along before long. They have some good rules of thumb about mixing panels.

    RUss
    [SIGPIC][/SIGPIC]

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    • #3
      Generally, the closer you can match, the better.

      First, disconnect from the charge controller, the string which has the broken panels. That will prove that your other 3 strings of 6 panels
      (Is this right, do I understand you had 4, strings of 6 panels, and one string had 3 panels damaged?) are indeed able to run the controller.
      Controllers need the proper voltage to start up, and will use all the current the solar gives it, so if everything else is good, it should start up, and just not deliver as much power.

      Now series strings, and parallel.

      Solar wired in series, will be limited to the lowest current panel (your 4.35A panels) So your new 6.5A panels will only produce 4.3 in that string.

      When you parallel , the lowest voltage panel (or string) will clamp the system voltage to the lowest. So if you added 3, 17V panels to your 3, 23V panels. you would produce, 120V Tied to your other strings, which are 138V, you will pull the whole array down to 120V, and loose a fair amount of power harvest.

      If you bought 8 new panels @ 17.1 you would get 136.8V, which is a VERY close match to the existing 138V, and you would have 3 spare panels from the orig mfg.

      But, fitting in the extra panels may be a pain. Maybe look at replacing the damaged string with 3, 45v panels, or some 70V panels.

      What I don't understand, why are you running such a high voltage array, you are pretty close to the controllers Vmax death input. One really frosty morning could fry the controller. (it's your controller , you look up it's max input voltage)
      Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
      || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
      || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

      solar: http://tinyurl.com/LMR-Solar
      gen: http://tinyurl.com/LMR-Lister

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      • #4
        Thanks for the feedback! The charge controller's max input voltage is 125V. Our record low is 17F in central Florida so I don't believe that's an issue. It was a grid-tied system packaged from Sunwize and came pre-configured. If I knew the modules' voltage was so rare I probably wouldn't have purchased it due to the problem that I'm facing now with replacements/expansion.

        I've located a SolarWorld 80W module that has a similar Imp rating of 4.48A (off by only 0.13A) although it's only 17.7V Vmp. So I am considering just replacing the 3 broken modules with 3 of these and spread the new lower Vmp modules across different strings in the array to minimize the difference. This would create 3 mixed strings with a total Vmp of 86.7V while the 3 original intact strings have 92V, a difference of only 5.7% resulting in a loss of ~200W from the original setup if my calculations are correct. I believe this would be the cheapest viable option that wouldn't require modification of my ground mount rack (since it currently just fits 24 modules).

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        • #5
          Ashima the current in any one string will be limited by the lowest Imp value in the string. So for example lets say you have a series string with 5 and 6 amp modules. and the total voltage is say 300 volts. Instead of having a 1800 watt string, you now have 1500 watts because of the 5 amp limitation.

          Sorry you had to learn the lesson the hard way, but that is why we always advise buying panels from manufactures from companies where solar is a micro fraction of their product line like Sanyo, Sharp, BP, and Kyocera. They have been in biz for decades and will be around tomorrow for warranty replacements and expansions. Once the government pulls it head out of its a$$ and quits subsidizing solar, all those solar companies that only make solar products will be out of biz the following day. It is already starting to happen.
          MSEE, PE

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          • #6
            Thanks again for all the helpful feedback. I'm grateful to have found such a resource, especially when my vendor has been unsupportive.

            Originally posted by Sunking
            Sorry you had to learn the lesson the hard way, but that is why we always advise buying panels from manufactures from companies where solar is a micro fraction of their product line like Sanyo, Sharp, BP, and Kyocera.
            Yes, another lesson learned. No more weird panels from weird companies.

            Originally posted by Mike90250 View Post
            First, disconnect from the charge controller, the string which has the broken panels. That will prove that your other 3 strings of 6 panels
            (Is this right, do I understand you had 4, strings of 6 panels, and one string had 3 panels damaged?) are indeed able to run the controller.
            Controllers need the proper voltage to start up, and will use all the current the solar gives it, so if everything else is good, it should start up, and just not deliver as much power.
            I had 6 strings of 4 panels and the damaged modules were spread out across the 6 strings (3 strings had 1 panel damaged each). I tried your suggestion and re-arranged the modules to fill in the gaps, now have 5 complete strings of undamaged 4 100W panels (23V 4A) with a 48V battery bank. However the Outback MX60 charge controller still says the input voltage is only 40V, 10V less than the battery bank so it stays in sleep mode even in full sun. Strangely, the battery bank read 58V two days ago and the input voltage was displayed at 46V but yesterday and today the battery bank read 51V and the input displayed 40V. I even tried taking out 2 of the 8 (6V) batteries and changing the system voltage in the MX60 to 36V. The input voltage on the MX60 then displayed 29V and the batteries at 38V.

            Tried the Outback forums and someone said I probably have more damaged panels or wiring, so used a digital DC multimeter to read the outgoing voltage from the array. Both in the PV combiner box and in the Epanel PV inputs it read 100V, with everything on and connected (so I assume under a load). I then checked the voltage and amperage of all 20 modules yesterday when we had full sun. I checked the voltage both disconnected and connected but only have a 10A multimeter so I disconnected the modules to check the amperage. All were around their rated values except one module that was consistently under by 0.2 - 0.3A. I also tested each string one by one in the PV combiner box, disconnecting the others, and got usual readings: around 100V and 4A. I also tried resetting to factory default settings in the MX60 and double checked proper float and absorb set points for my batteries.

            So there seems to be something else up. Is there anything else I could try or look for? Thanks so much again!

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