Morningstar 15 amp MPPT Charge Controller Voltage Limits

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Steve961
Steve961 Solar Expert Posts: 93 ✭✭✭✭
in Solar Beginners Corner #1
I was planning to buy a Morningstar 15 amp MPPT charge controller, but may have run into a snag. I would like to use it with three Unisolar US-64 panels in series. The open circuit voltage of the US-64 is 23.8 volts, and would be 71.4 volts in series. The maximum open circuit voltage of the Morningstar MPPT controller is 75 volts. In talking with Wind-Sun, they said that does not leave enough room for increased voltage in cold weather - it can get -20F in northern Wisconsin. They also said that might damage the controller and recommended I contact Morningstar.

Morningstar said I would need to calculate the increased voltage at low temperatures to be safe - something I don't know how to do. In further questioning he did say that small voltage overages should not damage the controller. He also said that the controller would shut down during any times of over voltage, but even on the coldest day that wouldn't be very long since the sun would warm the panels pretty quick.

Is this something I should worry about, or would I be fine?

Thanks.

Steve
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Morningstar 15 amp MPPT Charge Controller Voltage Limits

    -20F is fairly cold. You're not looking at 10% over, you're looking at 20% and on one nasty day 30%. It probably wasn't meant for 85 volts on the input, much less 92. All it takes is one high Voc day to do in the controller.

    I wouldn't do it.
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  • Steve961
    Steve961 Solar Expert Posts: 93 ✭✭✭✭
    #3
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    Re: Morningstar 15 amp MPPT Charge Controller Voltage Limits

    Thanks Cariboocoot. Not the news I wanted to hear, but I'll just have to work around it. Some options:

    1. Just use two panels for now, and keep the third as a spare. Or I could find a fourth panel and have two series arrays, although I'd probably be wasting a lot of extra power with that configuration.

    2. Get a Rogue MPPT controller, although that seems like overkill (both technically and monetarily) for my simple system.

    3. Stick with a PWM controller, although I am trying to avoid the big/expensive wire my 75 foot distance requires.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #4
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    Re: Morningstar 15 amp MPPT Charge Controller Voltage Limits
    Steve961 wrote: »
    Thanks Cariboocoot. Not the news I wanted to hear, but I'll just have to work around it. Some options:

    1. Just use two panels for now, and keep the third as a spare. Or I could find a fourth panel and have two series arrays, although I'd probably be wasting a lot of extra power with that configuration.

    2. Get a Rogue MPPT controller, although that seems like overkill (both technically and monetarily) for my simple system.

    3. Stick with a PWM controller, although I am trying to avoid the big/expensive wire my 75 foot distance requires.

    Always ready to jump in with a kludge for you:
    1. Use either a thermostatic switch or a voltage sensor to determine when the panel output will be high enough to exceed the tolerance of the Morningtar. Then
    2. Short out one of the panels in each string with a DC-rated relay until conditions improve.

    You will probably not lose too much power because the critical times will be as the sun just starts to hit the panels in the morning.
    You can decide whether going to shorter strings in parallel or applying a kludgey workaround to allow you to keep more in series is more attractive to you.
    Also to be considered is whether you want to accept the risk of the detector or relay failing, of course, since the shorter string will be intrinsically safe.
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
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    Re: Morningstar 15 amp MPPT Charge Controller Voltage Limits

    The US64 has a low Vmp as I recall, so you need two in series for either a 12 or 24 Volt system. I don't think they'd work well on a PWM controller; too much V-drop and you haven't got enough Voltage to charge with. 75 Feet is a pretty long run; it would be ridiculous to try that on a "12 Volt" panel @ 12 Amps (all three in parallel).

    Since you can still get these panels, putting four on the MS 15 MPPT is probably your best solution. Even so you would need 8 or 6 AWG to keep V-drop low for that long a run. You would also be running 256 Watts on a 12 Volt system which is more than that controller is rated for.
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  • Steve961
    Steve961 Solar Expert Posts: 93 ✭✭✭✭
    #6
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    Re: Morningstar 15 amp MPPT Charge Controller Voltage Limits

    inetdog:

    I actually started writing an idea similar to yours, but I thought it was too out there. Now that you've brought it up though, here goes. How about a two bank battery switch, like: http://www.solar-electric.com/miblseabases.html, that switches from three panels in series to two panels in series. I could "deactivate" the third panel during the winter months when my cabin is rarely used anyways.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
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    Re: Morningstar 15 amp MPPT Charge Controller Voltage Limits
    Steve961 wrote: »
    inetdog:

    I actually started writing an idea similar to yours, but I thought it was too out there. Now that you've brought it up though, here goes. How about a two bank battery switch, like: http://www.solar-electric.com/miblseabases.html, that switches from three panels in series to two panels in series. I could "deactivate" the third panel during the winter months when my cabin is rarely used anyways.

    Yes, that is entirely possible. You may not even need such a heavy switch, since the panels only put out about 4 Amps.
    Basically single pole, double throw to select the controller's input from "high Voltage" to "low Voltage". You would need to either locate the switch where the panels are or run an extra 75 foot wire to the controller.
    Or just disconnect one panel from the series when Winter rolls around.
    Forget to do it and your charge controller is toast.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #8
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    Re: Morningstar 15 amp MPPT Charge Controller Voltage Limits

    I prefer setup and forget--Rather than forgetting to reconfigure.

    Electronics are very sensitive and unforgiving to over voltage and over current conditions.

    I highly recommend that you bite the bullet and "do it right". Or, somewhere down the road, there will be an "oops" that takes out your charge controller.

    Running two series/parallel strings (over controller's maximum wattage rating at 12 volt output) won't hurt anything and will provide more power during the off-peak parts of the day (and during poor weather)--So, if a 4th panel is better for your budget--I would go for a 2x2 array configuration.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Steve961
    Steve961 Solar Expert Posts: 93 ✭✭✭✭
    #9
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    Re: Morningstar 15 amp MPPT Charge Controller Voltage Limits

    I have an update on this issue and would like some feedback. I was reading the charge controller manual and noticed that it claims there is high voltage PV protection built into the controller. The manual states "If the solar input open circuit voltage (Voc) exceeds the 75 volt maximum rating the array will remain disconnected until the Voc falls safely below the maximum rating."

    This is different information than I previously received from NAWS and Morningstar. I called Morningstar this morning, and the technical person I spoke to confirmed that their SunSaver MPPT controller does indeed have high voltage PV protection built in. He said as long as the voltage is less than 100 volts, the controller will be able to deal with it. I will still probably have a switch that disconnects my third panel for the coldest months, but it's nice to know the controller won't fry if I forget.

    Any other thoughts on this?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
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    Re: Morningstar 15 amp MPPT Charge Controller Voltage Limits
    Steve961 wrote: »
    I have an update on this issue and would like some feedback. I was reading the charge controller manual and noticed that it claims there is high voltage PV protection built into the controller. The manual states "If the solar input open circuit voltage (Voc) exceeds the 75 volt maximum rating the array will remain disconnected until the Voc falls safely below the maximum rating."

    This is different information than I previously received from NAWS and Morningstar. I called Morningstar this morning, and the technical person I spoke to confirmed that their SunSaver MPPT controller does indeed have high voltage PV protection built in. He said as long as the voltage is less than 100 volts, the controller will be able to deal with it. I will still probably have a switch that disconnects my third panel for the coldest months, but it's nice to know the controller won't fry if I forget.

    Any other thoughts on this?

    Methinks they may have had problems/complaints and made a much needed improvement. :D
    Some of the larger MPPT controllers have similar spec's: shut down at 'X' Voltage to avoid damage, but still have an upper limit. MX60's over the years have had changes which alter this "turns off/burns up" spec Voltage point.

    You can't beat talking with the manufacturer to get the latest information. But also check to see if there is a certain point in manufacture when such a change was made (revision '#.#', serial number after '#', manufactured after #/#/#).
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