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  • Panel volts exceeding MPPT controller

    Hey all. I'm just starting to get into the hobby.

    I received a few freebie solar panels from work, some silly amorphous stuff. The open circuit voltages for 2 different panels being up near 60 and 78 volts, and they put out a little less than an amp each. I would like to connect these 2 panels (and a couple others I'm going to make, from thin film metal cells I recieved)... End goal is to charge one massive 12v battery (maybe 100 Ah), to power an ATV winch every once in a while.

    I feel like I want to use an MPPT controller to get the most out of my screwed up panel arrangement. The Morningstar Sunsaver MPPT controller is rated at 75 volts open circuit, but one of my panels is 78 volts open circuit. Is there any allowable tolerance in these controllers? I really want to just buy this MPPT controller and be done with it, what are the realistic chances of "frying" the controller with 3 extra volts? I'm likely not pushing very many amps... maybe 4-5 amps total from my array when finally complete (sunsaver rated at 15A).


    Best Regards,
    Jason
    Mechanical Engineer

  • #2
    Originally posted by Jasonzebra View Post
    I really want to just buy this MPPT controller and be done with it, what are the realistic chances of "frying" the controller with 3 extra volts?
    Almost guaranteed to damage the controller.
    MSEE, PE

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    • #3
      so as a PE you must know that nothing is ever built to a zero tolerance, there is factor of safety, etc. Are you telling me that the tolerance on the controller is less than 3 volts? I'm wondering if they say 75 volts as a nominal, but it's really about 75 +/- 0.1 volt, or 75 +/- 5 volts at max current ... You know, of course you wouldnt want to list the ABSOLUTE maximum as the manufacturer because then people would creep up on it and you'd be liable. I know it's risky, but do you have any reason to believe that I wouldnt be able to apply 78V?

      Comment


      • #4
        Originally posted by Jasonzebra View Post
        Is there any allowable tolerance in these controllers?
        Often no. I once blew out a Blue Sky Energy MPPT controller that was rated for a 57 volt max by running it at 60 volts. When I talked to the manufacturer, they said "yeah, the devices we use actually top out at 55 volts, but the extra two volts is usually OK. But 60 volts will definitely blow them."

        I really want to just buy this MPPT controller and be done with it, what are the realistic chances of "frying" the controller with 3 extra volts? I'm likely not pushing very many amps... maybe 4-5 amps total from my array when finally complete (sunsaver rated at 15A).
        Very high - especially since on a cold morning it will be 13 extra volts, not just 3 extra volts.

        Comment


        • #5
          Originally posted by Jasonzebra View Post
          so as a PE you must know that nothing is ever built to a zero tolerance,
          Absolutely, but the voltage you quote is at 25 degrees C which is fine for the warmer months. But come winter you are now looking at possible 97 volts just at sun up.
          MSEE, PE

          Comment


          • #6
            So I guess I can't solve the problem by just throwing in a few diodes to drop the voltage... I would have been totally OK with blowing out 10 volts with some high power UV LED's or something on that one panel. It sounds like the only way to use this panel is by buying a $500 mppt that handles up to 150v

            Comment


            • #7
              Originally posted by Jasonzebra View Post
              So I guess I can't solve the problem by just throwing in a few diodes to drop the voltage... I would have been totally OK with blowing out 10 volts with some high power UV LED's or something on that one panel. It sounds like the only way to use this panel is by buying a $500 mppt that handles up to 150v
              The difference between Voc, especially cold, and Vmp can be quite significant. If you can determine the exact voltage versus current curve of your panel, you may find that you can in fact use a parallel zener diode or voltage regulator to draw just enough current from the panels to keep the voltage below the inverter limit. I would *not* recommend using a series diode which will cost you power even when the panel operating voltage is within the charge controller's limits. Nor should you put a fixed current load on the panels to keep the voltage down, since that will also waste power. I am not recommending this course, especially since the extra money for the more expensive CC may well be worth it for other reasons. But I do suggest that you do the computations based on your panel characteristics before making a decision.

              An MPPT controller should spend a very small amount of the time drawing low or no current from the panels while it is in bulk or high current absorb mode to the batteries. A PWM controller, on the other hand, will be seeing the Voc of the panels during the off-time of the current pulses to the batteries and so must be able to withstand the Voc of the panels even when it is actively charging the batteries.
              SunnyBoy 3000 US, 18 BP Solar 175B panels.

              Comment


              • #8
                Originally posted by Jasonzebra View Post
                I would have been totally OK with blowing out 10 volts with some high power UV LED's
                10 volts x How Many Amps = the wasted watts.
                MSEE, PE

                Comment


                • #9
                  Good point about the zener diode. I'm thinking it could actually be quite reasonable to create a shunt regulator for that panel. Regulate it to 70 volts. Just a quick search of digikey revealed several zener diodes with very high power ratings. I think this might be the best solution.

                  Comment


                  • #10
                    Originally posted by Jasonzebra View Post
                    Good point about the zener diode. I'm thinking it could actually be quite reasonable to create a shunt regulator for that panel. Regulate it to 70 volts. Just a quick search of digikey revealed several zener diodes with very high power ratings. I think this might be the best solution.
                    A shunt regulator? You should know better than that. You just as well use a PWM controller. Simple fact is any series or shunt regulator Input Current = Output Current. Do the math with that info 100 watts, 75 volt, 1.33 amps input, and 13 volts 1.33 amps output = 17 watts out to battery. Is that what you have in mind?
                    MSEE, PE

                    Comment


                    • #11
                      Originally posted by Jasonzebra View Post
                      Good point about the zener diode. I'm thinking it could actually be quite reasonable to create a shunt regulator for that panel. Regulate it to 70 volts. Just a quick search of digikey revealed several zener diodes with very high power ratings. I think this might be the best solution.
                      1) You won't find one that's rated to the powers you need. You'd need a power zener design (a zener with a bipolar or MOSFET follower) and a very large heat sink.

                      2) If you build it the controller won't work well. Controllers rely on seeing the full range of the panel and choosing the operating point based on that. If you clip the top end of the panel's voltage the controller will likely choose the wrong operating voltage.

                      If you really want to do all that work, just build a high power charge controller that will go to 150 volts. Open source designs for such devices are available.

                      Comment


                      • #12
                        Originally posted by Sunking View Post
                        A shunt regulator? You should know better than that. You just as well use a PWM controller. Simple fact is any series or shunt regulator Input Current = Output Current. Do the math with that info 100 watts, 75 volt, 1.33 amps input, and 13 volts 1.33 amps output = 17 watts out to battery. Is that what you have in mind?
                        I think that the "shunt regulation" being discussed is intended not to regulate the panel voltage down to the battery voltage but rather to regulate the maximum panel output voltage to keep it within the input voltage range of the MPPT controller. As long as the MPPT controller is pulling enough current from the panels to bring the in-circuit voltage below the maximum input voltage of the CC itself, no current will be flowing through that shunt regulator.
                        When the CC is not pulling any current (or minimal, as during float), the shunt regulator will be drawing far less than one amp to bring the Vloaded output of the panel from 97 volts at almost no current at first sun on a cold morning or 75 volts at full sun to the 70 volt input rating of the CC. Since the Isc of the panel is on the order of one amp, the loss in the zener system will be 70 volts times ~100ma, well within the power range of a large zener on a good heat sink.
                        SunnyBoy 3000 US, 18 BP Solar 175B panels.

                        Comment


                        • #13
                          so apparently there's 3 cheap options:

                          1. Find a single adjustable voltage regulator that can dissipate those amps. Kinda hard to find.

                          2. Hook up a couple voltage regulators in parallel with a feedback loop to ensure current is shared

                          3. Make a voltage regulator with a darlington follower to amplify the regulated current


                          there's DC-DC converters that cost an arm and a leg, and I'm not sure I understand the zener shunt regulator enough yet.

                          On second thought, it's nice learning about this stuff and all, but I'm considering an MPPT charger that can only handle 400 watts.... if I just buy the tristar MPPT for twice as much, I'd have the ability to put on up to 3200 watts... and no panel mumbo jumbo to fool with. It almost seems worth it.

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