Panel volts exceeding MPPT controller

JasonzebraJasonzebra Registered Users Posts: 13
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'm likely not pushing very many amps... maybe 4-5 amps total from my array when finally complete (sunsaver rated at 15A). Think I will fry the Sunsaver with 3 extra volts? Is there a better choice out there? Does anyone have any words of caution before I go ahead?


Best Regards,
Jason
Mechanical Engineer

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Panel volts exceeding MPPT controller

    Welcome to the forum.

    The V-max input of a charge controller is for the Voc of the panel(s); Voltage open circuit. The load doesn't enter in to it at this point. So putting 78 Volts to a controller with a V-max of 75 is a problem. If the temperature goes cold it gets worse, as the panel Voltage will rise further.

    Given the cost of any MPPT charge controller vs. a PWM type, to charge one 12 Volt 100 Amp hour battery you'd be better off ignoring the high Voltage panels and spending the money you would have put into the controller on buying a proper Voltage panel. Really.

    MS 15 MPPT: $225
    MS SunSaver 10 Amp: $46
    Difference: $179 (enough for a sizable "bargain" panel if you shop around)

    Cost of an MPPT that could take the 78+ Volts: a lot more.
  • JasonzebraJasonzebra Registered Users Posts: 13
    Re: Panel volts exceeding MPPT controller

    Thanks for the reply.

    I haven't purchased anything yet, so I'm definitely open to new ideas. I could consider going for a the Sunsaver PWM controller and getting a real 12v designed panel, but the idea of my ragtag array (using whatever I had, patching on to it panels from the assortment of solar stuff I have in my shop, etc) was quite appealing. I also liked the aspect of amorphous panels that allow them to generate power when not in direct sunlight (my array position is definitely less than ideal). It just seems like a waste to throw out the 2 panels I already have... is there anything I can do with them?

    ultimately, the whole project is going to be about making a little robinson crusoe treehouse with a winch elevator.

    Jason
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Panel volts exceeding MPPT controller

    You only need about 5 to 10 Amps to charge that battery; roughly >88 Watts. Lower rate charge will sort of work, but at the expense of the battery's life.

    The appeal of "using what you can find" is strong, but often not practical. As you've seen from this it sometimes ends up costing more to get the rag-tag equipment to work than just buying the right stuff to begin with.

    Problem with high Voltage panels is; the power is in the Voltage. Solar panels are actually a current source, so the high Voltage panels need the MPPT to make any power. A "normal" 12 Volt panel has a Vmp of about 17 to 18 and will put out its maximum current at anywhere from zero Volts on up to that point as required. You could even use "24 Volt" panels on a PWM controller and throw away half the power; at least the current will be there. But the high Voltage panels are a real pain to use.

    The amorphous quality is misleading. They do not put out more power in low light than crystaline panels. They always put out less per square area, but they don't need the light to be direct to achieve their maximum power. So they can consistently put out about 7% efficiency no matter how poorly aimed, whereas a properly aimed polycrystaline will put out twice that. Crystaline panels on the right kind of cloudy day show the same effect due to the scattering of light by the clouds: lower peak power production, but more consistent production throughout the day.

    If you were to take that "60 Volt" panel and connect it directly to the battery the Voltage would be dragged down to the battery point. So instead of it producing 60 Volts * 1 Amp = 60 Watts it would produce 12 Volts * 1 Amp = 12 Watts. A pretty horrible reduction in power and it wouldn't charge the battery.

    Unfortunately building the buck converter circuit that is the main ingredient in MPPT to convert the higher Voltage to lower with more Amps isn't easy. Although I'm sure there's someone on this forum who can tell you how to do it. :D
  • JasonzebraJasonzebra Registered Users Posts: 13
    Re: Panel volts exceeding MPPT controller

    I'm more wondering what the tolerance is on the supposed '75 volt maximum' on the MPPT controller, and what the mechanism of failure is. It's probably unlikely to overheat at the current I'd be driving with... and I know most IC's easily tolerate overload
  • BB.BB. Super Moderators Posts: 26,867 admin
    Re: Panel volts exceeding MPPT controller

    Ic's don't like over voltage at all. If an if ic is over voltage, it is stressed, it may fail immediately, in months or years, or never.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • JasonzebraJasonzebra Registered Users Posts: 13
    Re: Panel volts exceeding MPPT controller

    Alright, so what if I knocked down the 78 volts down to something lower, with a diode? I wouldn't be lowering the current, just the voltage.

    How much higher do you expect the 78 volt panel to climb to in the winter months? (I mean how much voltage would I have to knock off to stay under this 75 volt limit?)
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Panel volts exceeding MPPT controller

    In an open circuit condition (when the controller first connects) a diode isn't going to do much for you. With a load on it could take half a Volt or so (depending on the diode) out.
    78 Voc in a worst-case scenario will hit about 101 Volts. Definitely fried controller time.

    To really take the Voltage down you need to "preload" the panel so there is no Voc. The right amount of resistance (able to take full current) across the output so there is always some load on the panel and the Voltage is held below safe levels. Trouble is, that means some of the power goes to heat in the preload. It's easy to experiment with though. :D

    The Voc issue is something we deal with a lot up here, and even with standard panels and high Voltage controllers it can be a problem. MidNite's Classic series with the hyperVoc function is going to prove invaluable in these climes!
  • JasonzebraJasonzebra Registered Users Posts: 13
    Re: Panel volts exceeding MPPT controller

    Haha so I basically keep my 60 volt panel, throw out the 78 volt panel.... and go on with my life. I could see that.

    I may try to experiment, like you said, with some power resistors / diodes.... something that won't be a fire hazard. I think the easiest thing to do might be to create an array of diodes in parallel with some super low ohm pull down resistors.... Some UV LED's have like a 4.5V forward drop, so maybe if I found some high powered ones I could create like a 16 LED array. MAYBE that would let me use this panel in some kind of handicapped fashion.

    Edit - maybe it's less hassle to grind into the glass and short the last 10 strips or something...

    I know this all sounds like a waste of time.
  • DillDill Solar Expert Posts: 170 ✭✭✭✭✭
    Re: Panel volts exceeding MPPT controller

    or try to sell it on Craigslist and use any funds towards the panel you really want.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Panel volts exceeding MPPT controller

    Considering the low current potential of the panel, something with 1 Amp Zener diodes might work. V climbs above 72 say, diodes conduct and kill the power. So long as V stays below then the power would be fine.

    You really need the advice of some of the guys here who design circuits. I'm more of a "get the thing to work" technician. :D
  • JasonzebraJasonzebra Registered Users Posts: 13
    Re: Panel volts exceeding MPPT controller

    Thanks for the brainstorm. I started looking up zener diodes and it seems like, given a beefy enough diode, I could make a pretty good shunt regulator for the panel!!! Easily take this thing down to 70 volts maximum and just burn off the excess as heat. I checked digikey and they have diodes that will dissipate up to 50 watts.... wow.
  • Steve961Steve961 Solar Expert Posts: 93 ✭✭✭✭
    Re: Panel volts exceeding MPPT controller

    Jason:

    If you figure out a good way to do this, please share it with the rest of us. I have three panels that I want to run in series to my Sunsaver MPPT controller, and the max Voc could reach 88 volts in a worst case cold spell here. I was thinking of making a switch to remove a panel in the cold weather, but I would much prefer an automatic system such as this.

    Thanks.

    Steve
  • JasonzebraJasonzebra Registered Users Posts: 13
    Re: Panel volts exceeding MPPT controller

    Sounds good. I'm going to try and remember to talk to an electrical engineer tomorrow. At first glance it seems very easily doable with the only downside being, that big diodes cost big $. The 50w diode costs $30, a 10W diode costs $20, a 5W diode costs 53 cents..... so it might be interesting to see if I can do something like a few shunt regulators in parallel to share the regulation.. I don't know, but I'm sure something is possible. give me a few days.
  • JasonzebraJasonzebra Registered Users Posts: 13
    Re: Panel volts exceeding MPPT controller

    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.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Panel volts exceeding MPPT controller
    Jasonzebra wrote: »
    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.

    Maybe... You can't just mix and match any panels. It may be difficult to add capacity to the panels you have. It seems that you are going to too much trouble and expense to use the panels you have.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • JasonzebraJasonzebra Registered Users Posts: 13
    Re: Panel volts exceeding MPPT controller

    you cant use different panels with an MPPT controller? It makes sense that you wouldn't be able to take advantage of all the solar power, but isn't the MPPT controller designed to sweep through the IV curve and figure out where the sweet spot is?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Panel volts exceeding MPPT controller
    Jasonzebra wrote: »
    you cant use different panels with an MPPT controller? It makes sense that you wouldn't be able to take advantage of all the solar power, but isn't the MPPT controller designed to sweep through the IV curve and figure out where the sweet spot is?

    You can't use panels that are too different from one another on any type of charge controller.
    An MPPT controller can not make a 75 Volt panel and a 35 Volt panel work together, as it were. It can make up for a difference in panel Vmp vs. battery Voltage, but basically the same panel mixing rules apply to all charge controllers:
    Panels connected in parallel need to have close Vmp (best within 5%).
    Panels connected in serial need to have close Imp (best within 5%).

    The MPPT will figure out where the "sweet spot" is in respect to the array to battery for the amount of light available and charging point, but it can nae work miracles Cap'n! :D
  • BB.BB. Super Moderators Posts: 26,867 admin
    Re: Panel volts exceeding MPPT controller

    If you have too big of Vmp or Imp miss-match, you have multiple peaks and no guarantee which peak the controller will track on... And, for example if you have a Vmp=35 volts and another with Vmp=70 volts--There is no "ideal"--Any of the two to three peak picked, is still less than the true amount of available. power.

    Pmax is a relatively flat peak--So, it is possible to have a Vmp miss-match (or Imp) of ~10% (say Vmp=35 and a second Vmp=39 volts) and you will lose around 10% of the array power output maximum (not much in the grand scheme of things).

    If you have a 100 Watt Vmp=35 volt panel in parallel with another 100 watt Vmp=70 volts panel, you will have several peaks (none of which will be 200 watts from the array):
    • Vmp-controller = 35 volts; you will have ~100 watts+50 watts = 150 watt array output
    • Vmp-controller = 70 volts; you will have ~0 watts + 100 watts = 100 watt array output

    If you have Vmp somewhere between 35-70 volt, you probably will still have less than 150 Watt output).

    And which peak will the MPPT controller "track"--I don't know. And since it is not a "requirement"--there is no guarantee that vendors will have the same "solution".

    The "normal" solution is to have two MPPT controllers -- One running the Vmp=35 volt array, and a second controller running the Vmp=70 volt array.

    Of course, if you can get two 35 Vmp panels, put them in series, and then the string in parallel with the Vmp=70 volt array--Then you will have a true MPPT solution with 300 watt output (optimum Vmp-array=70 volts).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 4,276 ✭✭✭✭
    Re: Panel volts exceeding MPPT controller

    I wonder if run in parallel if the lower voltage VOC panel won't 'regulate' the higher voltage panel, similarly to how they work for peak VMP? Run in parrallel the CC only 'sees' the pair at the lower voltage.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Panel volts exceeding MPPT controller
    Photowhit wrote: »
    I wonder if run in parallel if the lower voltage VOC panel won't 'regulate' the higher voltage panel, similarly to how they work for peak VMP? Run in parrallel the CC only 'sees' the pair at the lower voltage.

    No, because it's Voc; a panel being a big diode doesn't present any load to another panel. Put the two together in parallel and what you measure as Voc will be the higher amount.
  • JasonzebraJasonzebra Registered Users Posts: 13
    Re: Panel volts exceeding MPPT controller

    so I just checked the difference between the 2 panels...

    panel 1:
    VOC: 78 volts
    VMAX: 61 volts

    panel 2:
    VOC: 61 volts
    VMAX: 47 volts

    definitely greater than 10% difference.... and then I think I was planning on stringing together a bunch of thin film metal cells... I have like 40 units of flexible 3.5v sheet that I would probably make into panel 3 which maybe could average between panel 1 and 2???
  • BB.BB. Super Moderators Posts: 26,867 admin
    Re: Panel volts exceeding MPPT controller

    If it is "cheap"--Why not try it... You saw a 2x different in panels could result in a 25% reduction in output (again, don't know which peak the controller will pick).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Solar Expert Posts: 3,121 ✭✭✭✭
    Re: Panel volts exceeding MPPT controller
    BB. wrote: »
    If it is "cheap"--Why not try it... You saw a 2x different in panels could result in a 25% reduction in output (again, don't know which peak the controller will pick).

    -Bill

    There is a very good chance that the controller will pick the wrong one. Since Vmp of one panel is close to and possibly higher than Voc of the other, as the CC starts to draw more and more current seeking the MPP, it will first find the MPP of the single higher voltage panel and will have no idea that it will be able to double the current by pulling the voltage down lower.
    If there is an overlap, with Vmp of the higher voltage panel below Voc of the second panel, ,the CC should see only one peak, somewhere above the Vmp of the second panel.

    On the other hand, if the CC tries to sample the voltage current profile over a wider range before zeroing in, it still may find the true maximum even with the first situation.
    SMA SB 3000, old BP panels.
  • JasonzebraJasonzebra Registered Users Posts: 13
    Re: Panel volts exceeding MPPT controller

    I never spend money on anything... no tv, cellphone, I do my own haircuts, I don't buy pretty much anything but tools. so I have the funds to throw away on something like this, but as is everything else, it's always hard to justify haha
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Panel volts exceeding MPPT controller

    I just reread this thread. Cariboocoot nailed it in the first response (post #2). For less than you spend trying to make your panels work, you could buy a properly matched panel and controller and generate more power. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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