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Thread: Panel volts exceeding MPPT controller

  1. #1
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    Default 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. #2
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    Quote 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

  3. #3
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    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?

  4. #4
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    Quote 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.

  5. #5
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    Quote 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

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    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

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    Quote 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.

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    Quote 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

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    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.

  10. #10
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    Quote 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

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