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  • MPPT vs. PWM

    Hello all,

    First post, been looking around for a few days, but couldn't find a close enough situation to get the advice I'm looking for.

    We have a 21' trailer, and I estimate we use 30-35Ah per day based on the 100Ah reserve (2 group 27's @ 12v), and needing to run the generator after 3-4 days. I'm looking at a 200W kit from Renogy, but the problem is here in NY, there will always be partial shade due to trees around the site. Best case, I leave the unit on the ground and move it around during the day.

    My thoughts are a MPPT with the panels in series would cut my current in half, but the higher voltage would keep the current flowing longer. 6A estimated X 7 hours = 42 Ah , vs. PWM where at least one panel might be in partial shade, and the time that I can keep the voltage above 14V is a big unknown..

    I'm probably missing a lot here, so if anyone has a similar set-up and could give me some insights, it would be greatly appreciated!

    Thanks
    Looney

  • #2
    In any light your panels will put out more than 14 volts. What you need to be concerned with is that they put out full amperage. That comes from proper orientation in full sunlight. Shadows are power killers. In series any shadow on either panel will kill the amperage of the whole string. Your best bet would be to find a location to mount your panels where they WILL get full, unobstructed sun. Use a MPPT controller and heavy enough gauge wire to carry the amperage whatever distance you need from the sunny location to your trailer. If this will be like more than , say 50', you might consider a third panel, in series, to raise the voltage higher making it easier to push the amps to your controller.
    2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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    • #3
      Almost certainly want MPPT because that will allow you to charge your 12v batteries from a higher voltage panel(s) without the large losses of PWM in same situation. Also I agree with littleharbor that using portable ground mount to position panel(s) where full sun is available is a big plus and worth considering, look into a single grid tie panel of around 230 - 280 w range for simpler mounting, lower per watt cost, etc., but either way one larger panel or 2 smaller you wont have any trouble running between 50 and 75 feet away at approx. 35v or higher, you only need 12 or 10 guage wire to stay with acceptable losses. so no big cost there, approx. $20 to $35 for the wire.

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      • #4
        Interesting... If I buy a 270W 24V panel, can I get away with a 15 amp controller?

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        • #5
          Some MPPT controllers allow you to overpower them and will limit their output to their rated amperage. 270 watts divided by 12 volts is 22.5 amps, so you could use a 15 amp controller that will accept excess input. I would recommend a 30 amp version myself. You might want to add a panel someday. Be careful when buying your controller. There are some inexpensive "so called" MPPT controllers out there which aren't what they claim. Look in the specs for either 100Voc. or 150 Voc. input limits across the board for any nominal battery voltage. While you're at it see if they allow higher input while limiting output.
          2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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          • #6
            I guess I'm not clear on the technology, Your saying the controller takes the input (24V@8.77A,) and uses a transformer to output 14V @ 15A? providing no load on the panel?
            The current is rated at the input, or output?

            https://www.amazon.com/RENOGY-Polycrystalline-Black-Solar-Listed/dp/B01MY0XX1I/ref=sr_1_17?ie=UTF8&qid=1498905712&sr=8-17-spons&keywords=24v+solar+panel&psc=1
            Last edited by Loony; 07-01-2017, 08:17 AM.

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            • #7
              Ample Sun 100 thin film 3.jpg The rated current is on the output.

              When the controller drops the voltage to what your battery is accepting, the amperage goes up proportionately i.e 100 watts is 100 watts, be it 12 volt at 8.32amps or 24 volts at 4.16 amps. I had some 100 watt thin film panels (click photo for larger image) rated at 1.29 amps @77 volts, Still 100 watts. The MPPT controller will step down that high voltage/low amperage to say 14 volts @ 7.14amps
              Last edited by littleharbor; 07-01-2017, 09:06 AM.
              2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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              • #8
                Ok... Thanks for your patience! Next question... Does the controller limit the output current to the correct limit without boiling the batteries? Since mine are in parallel, I can soak up more current, but how do I know what the limit is for the batteries? I can run 6 or 8 wire from the controller to the batteries, but the batteries are the limiting factor.

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                • #9
                  I just answered my own question... I have an estimated capacity of 200Ah, so I cannot exceed c/5, or 40 amps... C/10 is more desirable, and provides me with plenty of capacity for our usage.. Next, i'm going to research the MPPT controllers as I imagine you can customize the output to your battery capacity. THANK-YOU VERY MUCH!!!

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                  • #10
                    Here's a nice video from Morningstar. At about 32.45 minutes into the video there is a great explanation of the advantages of overpowering their MPPT controllers.

                    https://www.youtube.com/watch?v=nqjp4_y3WVY

                    2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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                    • #11
                      Originally posted by Loony View Post
                      Interesting... If I buy a 270W 24V panel, can I get away with a 15 amp controller?
                      Originally posted by Loony View Post
                      I just answered my own question... I have an estimated capacity of 200Ah, so I cannot exceed c/5, or 40 amps... C/10 is more desirable, and provides me with plenty of capacity for our usage.. Next, i'm going to research the MPPT controllers as I imagine you can customize the output to your battery capacity. THANK-YOU VERY MUCH!!!
                      You want to make sure and remember your batteries also have a minumum current they need to reduce sulfate, c/12, so if you are running a 200ah lead acid battery bank it will need to see at least 18 amps fairly often to stay healthy. Victron has some good basic MPPT controllers that are very good and not too expensive, Morningstar, outback, and the other big names of course are excellent as well, may have a few more bells and whistles, etc. depending on your needs and budget.

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                      • #12
                        PWM Output Current = Input Current
                        MPPT Output Current = Panel Wattage / Battery Voltage.

                        So take say 2 panels. One panel is a standard 12 volt battery panel with 36 cells and Vmp = 18 volts, and Imp = 5.5 amps.. Second panel is a 200 watt 24 volt panel with Vmp = 36 volts and Imp = 5.5 Amps.

                        Now you have a 12 volt battery and your choice of PWM or MPPT.

                        So you cheap out and by 10 Amp MPPT Controller. With the 100 and 200 watt panel you get 5.5 amps out into a 12 volt battery. 12 volts x 5.5 amps = 66 watts from either a 100 or 200 watt Panel.

                        Or you spend a few extra bucks on a 20 amp MPPT Controller.

                        With the 100 watt panel;Output Current 100 watts / 12 volts = 8.3 amps. 12 volts x 8.3 amps = 100 watts.
                        With a 200 watt panel you get 16.6 amps or 200 watts out.

                        It is a NO BRAINER decision,.

                        MSEE, PE

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                        • #13
                          Thanks a lot!, I was hung up on the battery voltage limiting the current.. Now I need to search out some deals..

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