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  • Series, parallel or both for panel wiring

    Hello everyone!

    I live in the US Virgin Islands in the Caribbean. Still no grid power where I live since Hurricane Maria so I am setting up my little cabin to be off grid. I have purchased (4) 270W LG solar panels at 28.8 V and 6.8A. I plan to use the Midnite Kid charge controller to charge a 24V battery system using 4 Trojan T-105RE batteries. The main loads on the system will be 4 - Caframo 24V fans, 4 - 24V lights and a 24V solar refrigerator from Eco Solar Cool.

    My main question is wiring my solar panels in series or parallel or a combination of both. I was leaning towards series to have maximum voltage and minimize wire size. However, when I looked at the Midnite Kid documentation, they show a 4 panel configuration at 24V with two panels each wired in series and the two strings wired in parallel. See attachment. If I went this route then I would need to buy a combiner box which seems unnecessary and wouldn't be needed if they were all in series.

    Can someone please advise if following Midnite's wiring diagram for my panels is better than wiring them all in series and going straight to a circuit breaker disconnect box upstream of charge controller?

    I really appreciate the help and have enjoyed this forum and its wealth of information.

    Warm vibes from the Caribbean!
    Attached Files

  • #2
    Yes, you need 2x2 because the Voc on four in series is too high.

    You don't need a combiner box, MC4 Y-connectors are fine when you only have two strings.
    CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

    Comment


    • #3
      Thanks sensij, that makes sense.

      Comment


      • #4
        As a general rule of thumb, the greater the voltage difference between the panels and the batteries, the harder the controller has to work to transform the voltage. In the real world this results in slightly lower efficiency and more heat generation. Since you're operating in a hot semi-tropical area, excess heat should be a concern. Looking up the LG specs, I see a Vmp of 31.5 and 8.6amps. Are the numbers you're quoting NOCT values? That's very appropriate for hot, tropical areas. Running your panels in pairs at 57VDC, with 12 gauge copper wire you're only loosing about 1 volt per hundred feet of wire (<1%), which I would consider insignificant. How far from the controller will the panels be positioned?

        You don't necessarily have to have a combiner box, although they make life easier. You can add inexpensive in-line fuses instead.

        You might take a look at Renogy's 40amp controller that can operate at 24V. Forty amps would let you add another pair of your LG panels for future upgrading. https://www.renogy.com/renogy-40-amp...50-back-order/. With three strings though, you should have a combiner box.

        It looks like you will only be running DC loads with this system. Do you have any plans on adding an 24V inverter for AC power?

        Comment


        • #5
          Originally posted by MichaelK! View Post
          You don't necessarily have to have a combiner box, although they make life easier. You can add inexpensive in-line fuses instead.

          You might take a look at Renogy's 40amp controller that can operate at 24V.
          Fuses (or any OCPD) offer no protection when there are only one or two strings. They only need to be used when there are at least three strings.

          40 A is an aggressive charge on the T105RE batteries, and I don't think Renogy's controllers have a user settable limit below the output rating. The 30 A Kid is well matched to the rest of the system, and it's ability to work with a 48 V battery builds in a path for upgrading later, if needed.
          Last edited by sensij; 12-15-2017, 12:06 PM.
          CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

          Comment


          • #6
            Originally posted by MichaelK! View Post
            You don't necessarily have to have a combiner box, although they make life easier. You can add inexpensive in-line fuses instead.
            Ignore that advice. No fuses are required or needed. Just a waste of money and something else to fail.

            MSEE, PE

            Comment


            • #7
              Originally posted by Sunking View Post
              Ignore that advice. No fuses are required or needed. Just a waste of money and something else to fail.
              Thanks Sunking. I used your sticky posts to size mys system so I value your advice.

              Would you recommend a combiner box?

              Also do you think a surge protection device is necessary for an off grid mostly DC system?

              Comment


              • #8
                Originally posted by tang tonic View Post

                Thanks Sunking. I used your sticky posts to size mys system so I value your advice.
                You are more than welcome. Are you on St Thomas? I was there as part of the Hurricane recovery. When I left most power was restored. Suprised you are still without power.

                One thing I am confused about, not really important, is you stated you have 4 x LG 270 watt panels. However your numbers do not jive, and if correct a Midnite Solar Kid Charge Controller i snot quite large enough. What I mean is if Vmp = 28.8 and Imp = 6.8 amps (28.8 x 6.8 = 195 watts) would be a 200 watt panel, 800 watts total. Understand what I mean?

                Whether or not they are 200 or 270 watts is not a big deal other than 800 watts does not equal 1080 watts. However if they are 270 watts with the Kid Controller, you will be clipped to 800 or so watts depending on Vmp input voltage

                Originally posted by tang tonic View Post
                Would you recommend a combiner box?
                Not for two parallel strings, but I do not see a problem using one if you wish.

                Originally posted by tang tonic View Post
                Also do you think a surge protection device is necessary for an off grid mostly DC system?
                DC has nothing to do with it. Think about that. Is a tree AC or DC? Lightning does not care if it is a tree, your mother-in-law, flagpole, telephone pole, tower or whatever. If your system is properly bonded, lightning will not be a problem with a nearby strike. if you have a direct strike no TVSS is going to save your equipment. Thus is why USA electrical codes do not require TVSS or LPS. If you are not connected to the grid in any way, shape, or form TVSS or LPS gains you nothing.
                MSEE, PE

                Comment


                • #9
                  Edit Note:

                  Got to thinking and looked at MS KID specs. Still confused on your panels being 200 or 270 watts. Gather your panel specs and use MS Kid Sizing tool and see what it comes up with. With your warm climate you can do things colder climates cannot do. The KID is most efficient with panel Vmp of 90 to 100 volts with a 24 volt battery. Check the graphs on page 49 of manual.
                  MSEE, PE

                  Comment


                  • #10
                    Originally posted by Sunking View Post

                    You are more than welcome. Are you on St Thomas? I was there as part of the Hurricane recovery. When I left most power was restored. Suprised you are still without power.
                    I am on St. Croix. My wife is from St. Thomas and her parents live there. Both islands are at about 60% give or take but both her parents and us are not part of that 60%. Our location on St. Croix in particular means we will not be re-energized until February or maybe even March.

                    Originally posted by Sunking View Post

                    One thing I am confused about, not really important, is you stated you have 4 x LG 270 watt panels. However your numbers do not jive, and if correct a Midnite Solar Kid Charge Controller i snot quite large enough. What I mean is if Vmp = 28.8 and Imp = 6.8 amps (28.8 x 6.8 = 195 watts) would be a 200 watt panel, 800 watts total. Understand what I mean?

                    Whether or not they are 200 or 270 watts is not a big deal other than 800 watts does not equal 1080 watts. However if they are 270 watts with the Kid Controller, you will be clipped to 800 or so watts depending on Vmp input voltage
                    Yes, I think I understand and correct I have (4) 270W LG panels. Vmp = 31.7 and Imp = 8.52

                    The panels were a really good price and I thought three would be a bit undersized so went with four. I am considering the Midnite Kid charge controller so the charging rate would not exceed the C/8 for the 225 Ah batteries. I liked the Midnite controller because I could get it in a marine version which might last longer in the tropical climate here and close proximity our property is to the ocean. I also figured I would not get 100% of the power they were rated for because of the warmer climate here.

                    I have not purchased the charge controller yet, so if there is a better alternative, I am all ears.

                    Originally posted by Sunking View Post

                    Not for two parallel strings, but I do not see a problem using one if you wish.
                    Ok


                    Originally posted by Sunking View Post
                    DC has nothing to do with it. Think about that. Is a tree AC or DC? Lightning does not care if it is a tree, your mother-in-law, flagpole, telephone pole, tower or whatever. If your system is properly bonded, lightning will not be a problem with a nearby strike. if you have a direct strike no TVSS is going to save your equipment. Thus is why USA electrical codes do not require TVSS or LPS. If you are not connected to the grid in any way, shape, or form TVSS or LPS gains you nothing.
                    The reason for the question, and now I see the silliness in asking about a DC system is because I had read in another thread that with a direct lightning strike you are SOL anyways. I should have just said for "off-grid" rather than DC. So since the system will be off-grid I guess this component is not necessary.

                    Thanks for the help!

                    Comment


                    • #11
                      Originally posted by Sunking View Post
                      Edit Note:

                      Got to thinking and looked at MS KID specs. Still confused on your panels being 200 or 270 watts. Gather your panel specs and use MS Kid Sizing tool and see what it comes up with. With your warm climate you can do things colder climates cannot do. The KID is most efficient with panel Vmp of 90 to 100 volts with a 24 volt battery. Check the graphs on page 49 of manual.
                      According to the sizing tool I need 1.3 kids and the array power is "excessive". Any recommendations on a slightly larger capacity charge controller?

                      Comment


                      • #12
                        Originally posted by tang tonic View Post

                        According to the sizing tool I need 1.3 kids and the array power is "excessive". Any recommendations on a slightly larger capacity charge controller?
                        Yes Sir. Try a Morningstar TS-MPPT-45 amp model. At 1080 watts is going to require at least 40 amps. At 24 volts the Morningstar is good to 1200 watts.

                        Lastly and I think I understand why you are looking at T-105's because they are available to you. However they are a bit undersized for 1080 watts of panels. At 24 volts ideal size is 400 AH or almost twice the capacity. Check and see if there are any 350 to 500 AH 6-volt batteries available to you. Last choice would be 8 x T-105's but that requires parallel strings which you would like to avoid.

                        You can get away with the T-105's but hitting them with 40 amps is a bit too much. Give it some thought.
                        Last edited by Sunking; 12-18-2017, 02:24 PM.
                        MSEE, PE

                        Comment


                        • #13
                          Ok got it. Maybe what I should do is use three of the 270W panels connected to a 30A charge controller and use the fourth panel to power a 24V well pump since I do have a well and am looking to power it with solar.

                          I already purchased the batteries and yes the reason was availability and could not justify going to the L-16 size for the relatively small load I will have on the system.

                          Comment


                          • #14
                            Originally posted by tang tonic View Post

                            According to the sizing tool I need 1.3 kids and the array power is "excessive". Any recommendations on a slightly larger capacity charge controller?
                            Opinions vary, but I would let your actual consumption be the strongest factor in sizing your system. If you are confident that 4x T105RE's have enough storage to meet your needs, the 30 A mppt CC is a good match to them. Oversizing the array will not hurt your system, and provides some insurance against several consecutive days of cloudy weather that the Kid sizing tool doesn't consider. "Clipping" at 800 W is not a problem if you don't *need* the energy that is being clipped, and generally speaking, the only days you will experience that clipping are the kind of ideal solar days in which your battery will be easily charged anyway. On those days, even with a 40 A CC, that power would get clipped anyway because the battery will be in absorb and not taking the maximum power the array can generate.

                            You will be glad to have that extra power on less ideal days.
                            Last edited by sensij; 12-18-2017, 02:46 PM.
                            CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

                            Comment


                            • #15
                              Originally posted by tang tonic View Post
                              Ok got it. Maybe what I should do is use three of the 270W panels connected to a 30A charge controller and use the fourth panel to power a 24V well pump since I do have a well and am looking to power it with solar.

                              I already purchased the batteries and yes the reason was availability and could not justify going to the L-16 size for the relatively small load I will have on the system.
                              No, if you go with 3 panels, you have to put them in parallel, and the Vmp for 60 cell panels in parallel is too low to support a 24 V bank (and you need fuses, heavier wire to carry more current, etc).

                              2x2 is fine with the 30 A controller, really.

                              (Edit: sorry, 3 in series is ok here, I had been thinking 100 V max input when I made this post, but the Kid accepts 150 max. everything else I've written here is fine).
                              Last edited by sensij; 12-18-2017, 05:15 PM.
                              CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

                              Comment

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