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  • an unusual series/parallel panel wiring configuration...

    Hi,

    I'm new here, but not new to off grid PV.
    I'm upgrading my existing PV system.

    Currently I have 4 * 135w (12v. nominal) panels, (2 * 24v 270w series strings) that are running through 2 * 15 amp circuits
    from rooftop to a cheapo 24v/30 amp charge controller to a 24v battery bank.
    Very simple, all good.

    I want to add 3 * 250w (24v nominal) panels and replace the cheapo controller. with a 45 amp MPPT controller.
    I want to utilize the existing roof to controller wiring.
    To stay under 45 amp controller capacity and the 15 amp roof circuits, I need to bump up to 48v strings.
    The new system would be:
    4 * 135w = 540w in one 48v string on one of the 15amp circuits
    3 * 750w = 750w in one 48v string on the other 15amp circuit
    The unusual part of this, and my question is...
    Is there any reason not to parallel 2 of the 250w 24v panels, and then series them to the third 250w panel?

    Thanks, Eric






  • #2
    Yes. There is good reason not to do that. Series connected elements must carry the same current while parallel connected components must have the same voltage.
    By buying three more panels you are shooting your self in the foot leaving no good options except to put all seven in parallel.
    Does your MPPT CC have two separate MPPT inputs?
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

    Comment


    • #3
      Thanks for your input.
      The controller is a single input Tristar MPPT 45.
      As best as I understand things I can't parallel all 7 as 4 are 12v and 3 are 24v.

      I know it's an unusual situation.
      I was recently gifted the 3 new panels and CC and my budget is limited.

      I could leave one of the new 250w/24v panel out of the equation for now.
      That would leave the 4 * 135w/12v series-ed to 48v on one circuit
      and 2 * 250w/24v series-ed to 48v. on the other circuit.

      Do you agree that this is a reasonable solution until I get another 250w/24v panel?

      Comment


      • #4
        Originally posted by ericm808 View Post
        Thanks for your input.
        The controller is a single input Tristar MPPT 45.
        As best as I understand things I can't parallel all 7 as 4 are 12v and 3 are 24v.

        I know it's an unusual situation.
        I was recently gifted the 3 new panels and CC and my budget is limited.

        I could leave one of the new 250w/24v panel out of the equation for now.
        That would leave the 4 * 135w/12v series-ed to 48v on one circuit
        and 2 * 250w/24v series-ed to 48v. on the other circuit.

        Do you agree that this is a reasonable solution until I get another 250w/24v panel?
        You need to know the exact rated Vmp and Imp of each type of panel, not just the nominal voltage, to make the best decision.
        But you could parallel two sets of 12V panels and put the five total 24 units (two strings and three individual panels) in parallel.
        That parallel could be split up onto two different sets of wires if the existing wires are large enough to handle the current of two or three "panels".
        That way you would use all of your panels, but be working at a lower voltage and higher current, producing greater losses in the wiring.

        Your idea of one string of four and one string of two will also work, but will leave out the production of one panel.

        I would not use any arrangement that put old and new panels in series until I knew their Imp values.
        SunnyBoy 3000 US, 18 BP Solar 175B panels.

        Comment


        • #5
          250w panel Vmpp=30.7v, Impp=8.15a.

          135w panel Vmp=17.7, Imp=8.37

          My concerns in the 24v. config are:
          - running the 3*250w through a 20amp breaker, and
          - running all 7@24v through the 45 amp mppt controller.

          I appreciate your support very much.

          btw...the 2 20amp circuits off the roof are about 50' runs of #8 wire aluminium.

          Comment


          • #6
            You could use the old, existing controller for some of the panels, and use the MPPT for others.
            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

            Comment


            • #7
              interesting option...to the same battery array?

              Comment


              • #8
                Sure, you can have several charging sources feeding a single battery bank. I have a Midnight Classic, Morningstar MPPT and a Generator that all charge the bank on cloudy days - all at the same time,
                But a PV array can only go to 1 Charge Controller, You can use 2 arrays and 2 controllers (what I do with my 3Kw and 2Kw arrays) to one battery bank, But you can't hang 2 controllers onto one array.
                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

                Comment


                • #9
                  interesting...
                  Now I am wondering if I can series the 3 250w/24v panels at 72v to the mppt controller
                  and keep the 4 135w/12v at 24v to the old controller...?

                  btw...briefly checked your tinyurl link...fascinating world.

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