High voltage alarm

pictonrosepictonrose Registered Users Posts: 10 ✭✭
We added more panels and a second outback flexmax 80. We have panels wired in series/parallel. Panels are 245w, 30 v, 8 a. So I did the watts law calculations. 

120v x 32a = 3840w

The flexmax 80 specs says 150 VDC,  how do I convert voc to vdc?
We are getting high voltage alarm. Where have we gone wrong? 


  • BB.BB. Super Moderators, Administrators Posts: 31,425 admin
    edited February 21 #2
    Voc is the voltage from the solar panel(s) with no loads... Need to look at the exact specifications for your panels, but a "typical" Vmp~30 volt panel (voltage maximum power) has a Voc~37 volts (voltage open circuit).

    The FM80 (and a number of other MPPT charge controllers) have a maximum solar panel input voltage of 150 VDC maximum (to protect the input transistors from over voltage). How many panels you connect in series sets Voc-array and Vmp-array voltages.

    The other "gotcha" is that Vpanel rises in cold temperatures, and falls in hot temperatures (with "standard test temperature" ~77F/25C at the factory/marketing specifications).

    In warm/hot weather, the solar panel cell temperatures are (roughly) 20C hotter than ambient temperaturess. So "hot panels" have the Vmp-hot fall, and you need to make sure that Vmp-hot > Vbatt+ a couple degrees C (wiring and controller drops). Typically, we suggest that you take Vbatt * 1.3 = Vmp-array-std voltage. If you have a 48 volt battery bank:
    • 60 volts charging * 1.3 "hot array" fudge factor = 78 Volts Vmp-array-std suggested minimum for an MPPT controller
    For cold conditions, your specific condition... Voc rises when temperatures fall... In sub freezing conditions, the Voc-cold can be quite a bit higher. The math is not hard, but it is nice to have it automated (to avoid mistakes, give you multiple array configuration options, etc.). Outback's downloadable calculator is:


    I like the on-line tools--I don't have to download everybody's calculator and fire up the windows computer to run (typically use a Chromebook for posting). Midnite has a nice couple of websites. This one is for their "classic" larger controllers:

    I will make up some roughly accurate numbers for your array. And guessing you are at 0F (-18C) 245 Watt, Vmp-30.6 Voc=37.6, Imp=8. amps, Isc=10. amps, 4 panels in series:


    Rated PV Array Power:980Watts
    Anticipated Array Power @ 40C:914Watts
    Rated PV Array Current:8Amps
    Battery Charging Current @ 57.6 V:17Amps
    VMP (Maximum Power Point Voltage) :122.4Volts
    VOC (Open Circuit Voltage):150.4Volts
    VMP @ -18 C°:146Volts
    VOC @ -18 C°:171.6Volts

    At this point, it appears that your 4x panels in series is way to high of voltage--Especially at -18C... Your first fix (assuming I have the panel panel numbers close to correct, and your location/temperature (somewhere around Orrillia Ontario?)... A 3 string system would be:

    Rated PV Array Power:735Watts
    Anticipated Array Power @ 40C:685Watts
    Rated PV Array Current:8Amps
    Battery Charging Current @ 57.6 V:12.8Amps
    VMP (Maximum Power Point Voltage) :91.8Volts
    VOC (Open Circuit Voltage):112.8Volts
    VMP @ -18 C°:109.5Volts
    VOC @ -18 C°:128.7Volts

    Two panels in series is too low of working voltage for a 48 volt battery bank (but OK for 12 or 24 volt bank).

    And you would put 3 series and up to 8 parallel strings (at first glance).

    Details do matter--Please feel free to correct my guesses/assumptions if you need more help... But the first thing is to get to 3 series panels. Many of the MPPT controllers will record the max input voltage and void your warranty if you exceed their ratings (and obviously, damage can easily occur).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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