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  • Mismatched solar panels & MPPT sizing

    Hi there,

    I just purchased a sailboat and its electrical system was put together in questionable ways..

    It came with an 815W array of mismatched PV panels, linked to a Xantrex C40 (40A) PWM charge controller, charging a 1030Ah 12V lithium battery bank.

    First question: can mismatched panels damage the controller or reduce array efficiency in any way?

    Second, I'd like to replace the PWM controller with a Victron MPPT controller (or the like) to boost efficiency... how do I calculate the total voltage/amp rating for the controller given the differing panel specs? Do I simply add the VMP/IMP/etc values together? I guess this would also depend on whether I wire the panels in series or in parallel... suggestions?

    The run from panel to controller is relatively short, but would it still be best to wire the panels in series (to 96V) then have the controller step it down to 12V?

    The panel specs are as follows:
    3x Sun Electronics 95W
    (VOC21.2V, VMP17.4V, IMP5.46A, ISC5.99Am)
    3x Sun Electronics 90W
    (VOC21.2V, VMP17.5V, IMP4.57A, ISC5.6Am)
    2x Sun Electronics 130W
    (VOC24.1V, VMP19.8, IMP6.55A, ISC7.11Am)

    Thanks in advance for your advice!


  • #2
    Your panel wiring should be grouped by panels that will have sun or shade at
    the same time. Do not put in series, panels from different groups. Bruce Roe

    Comment


    • #3
      In a sailboat, because of uncertain sun, shadow, shadow from stays and lines, wiring panels in parallel is most efficient, generally.

      Your panels are well matched in voltage, which is good for parallel operation. You can figure your "nameplate" harvest would be 14V x (add all the IMP amps up)

      This would be problematic trying to put into series for a MPPT because in series, the amps must be closely matched, and the lowest panel (the 4.7A) would throttle all the others to 4.7A all the time.
      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


      • #4
        Originally posted by opfrei View Post
        how do I calculate the total voltage/amp rating for the controller given the differing panel specs? Do I simply add the VMP/IMP/etc values together?
        For series configurations, you'd want the total Voc (open circuit voltage) of the series string to never exceed the controller's max voltage input spec. Also factor in that the panel's actual voltage may slightly exceed Voc spec in cold weather.
        Last edited by sdold; 09-03-2019, 07:10 AM. Reason: Added an apostrophe

        Comment


        • #5
          Thank you for your responses!

          So it sounds like connecting the panels in parallel is the best option on a sailboat, given the variable shade and also to avoid throttling.

          As for the math, my understanding is that when connecting 8 panels (of differing specs) in parallel, I should average the panels' VOC (~21.9) & VMP (~18) and total all of their IMP's (43.2A) & ISCs (49A).

          According to the Victron calculator, I should choose a 150V / 60A controller. Does this seem right to y'all (and not overkill)? Any other controller recommendations.

          I plan to take these panels into both extremes of temperature as well, so there's that to consider as well.

          Lastly, the run from panel array to controller will be under 10', so which size wiring would you recommend?

          Comment


          • #6
            I would assume the panels are located about the boat, with the hope that 1 or 2 may have good sun, no matter what the orientation of sun, mast & stays is ?
            And with panels in parallel, you need protection in case one panel fails, and the other 8 start dumping raw power into it. is it better with a boat, to have a exterior fuse at each panel, or to run all the leads into a common, interior combiner box, where it's easier to work with, and use a clamp-on amp meter, to compare power readings (troubleshooting) being able to access all 8 panels from 1 junction box.
            Combiner Box (the page there also has links to both fuses (unable to remove from an energized circuit) and switch rated DC breakers that fit the box.
            Your panels likely have a sticker on the back listing the amp requirement for the Series Fuse, and with different panels, you will have several different specs. The breakers, come in many different amp ratings, so they are easy to match up.
            With about 5A at each panel, you would be fine using 18 ga wire from each panel + & - , wired to the Combiner.

            Here's where to "fudge a bit". It's unlikely that every panel will be fully illuminated at the same time, while the batteries are low. So while you "could" have 49A, it's unlikely.
            It's also unlikely a dive boat with 40 people sleeping on it, would catch fire and burn at anchor, but one just did off the coast of Santa Barbara this last week. +30 dead from that. So maybe it's best to use a 60A PWM controller .

            The Trace/Xantrex C-60 is a rugged beast. http://www.xantrex.com/power-product...ler_solar.aspx (conformal-coated boards)
            There is also the Morningstar Tristar TS-60M https://www.morningstarcorp.com/products/tristar/
            and the Midnight Brat 30A (use 2 for redundancy) http://www.midnitesolar.com/products...ers%20-%20Brat
            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
              Yes, what a tragedy... Fusing each panel consequently sounds like a must and I like the idea of a combiner box down below for ease of monitoring individual outputs.

              I already have a Xantrex C40 PWM controller so was thinking of supplementing it with an additional controller for redundancy (also a great idea on a boat) and to have 2 separate strings of panels, such that the 6x 90-95W panels would be connected in parallel to the C40 and the remaining 2x 130W panels would feed to a separate 20A controller. How does that sound?

              Also, I see you are recommending PWM controllers, whereas I thought MPPT were more efficient... Could you elaborate please?

              Comment


              • #8
                The efficency comes from using high voltage DC , in the 40 - 60V range. Wire like panels in series, but the MPPT controllers need the panels to be 1.3 x max battery voltage. so a 12V battery EQ'd at 16V needs at least 20V to reliably run, Add some heat and as the panel warms in the sun it's voltage drops. But if one panel becomes shaded on a boat, it's output shuts down, and you loose that needed voltage. And each string needs it's own MPPT, so you would need several expensive MPPT controllers, for very little gain. Panels in parallel, don't drag any other panels down, the shaded ones just stop contributing.

                MPPT controllers work like "smart transformers". They convert high voltage, low amp PV strings, into battery voltage, high amp, with high efficiency.
                Watts input = Watts output (minus a couple % for losses and overhead)

                PWM controllers simply switch panels on and off. Amps in = Amps out. A 40V 5A panel (200w) would only provide 5A to charge a battery. MPPT would convert that to 14V and 14A, and have little loss in the cables.

                When you get your bill for the 150' of 18ga wire (you are using marine tin plated copper wire right) you will understand more about using 1 lightweight cable instead of several heavy cables
                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
                  Great feedback, thank you. As for wiring, always tinned on a boat of course, what is the max voltage drop % allowed/recommend for panels to controller?

                  Comment


                  • #10
                    Panels to inverter, is generally 3%, panels to controller, 1.5% would be ok
                    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

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