Mismatch Panels in Series

Suntrekker
Suntrekker Registered Users Posts: 146 ✭✭
in Advanced Solar Electric Technical Forum #1

I’d like to add one panel to the array!  New panel Vmp:  40.6 Volts, Imp:  9.61 Amps, 390 Watts.

Old panel Vmp:  34.2 Volts, Imp:  9.37 Amps, 320 Watts.  

If three panels are in series, theoretically the array (under load) Vmp:  109 Volts and Imp:  9.37 Amps?  

Unfortunately the older panel is discontinued so a mismatch.  If there’s anything else to consider, please let me know.  Thnx!


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  • BB.
    BB. Super Moderators, Administrators Posts: 33,631 admin
    #2
    Adding panels in series... Need to make sure Voc & Vmp of the "new" array matches the connected devices (MPPT charge controller, Grid Tied inverter, etc.).

    The next check, the "ideal" match of all panels are Imp=9.37 amps +/- 5% (+/- 0.47 amps). Less ideal, but still works well would be +/- 10% Imp match (I.e., +/- 0.94 amps).

    5% panels would be Imp~8.9 amps to 9.84 amps).... 10% would be a larger range of acceptable Imp ratings.

    You "don't care" about Vmp/Voc of added panels as long as the total array Vmp-array and Voc-array are within the rating of the down stream device.

    You could, for example you Vmp18 volt or Vmp=30 volt, etc. panels in series too (or several of them in series) as long as you had the Imp +/- 5-10% rating on each series panels.

    More or less, the Imp-array will roughly be the Imp of the "lowest" Imp-panel in the series string.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    edited July 8 #3

    Nice Bill!  I really appreciate the Forum’s help! 

    I came up with these numbers but will check.  The mismatch panels are calculated separately to determine array Voc and Vmp.  The SCC is an Outback FM60, startup limit 145V (150V max). 

    Though I padded high and low temps, the Voc is close to the SCC 145V limit.  And Imp mismatch range is within 5% 👍

    Array Voc:  141.19   Array Vmp:  88.4

    SCC Voltage Limit - 145 (150 max)

    Thnx in advance for any suggestions!


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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    edited July 8 #4
    …another option is a 2S2P array to reduce the Voc.  But I prefer a 3S array and call it a (sunny) day.  :)

    I found another panel that may reduce the array Voc in a 3S configuration.  I need to research it, hopefully on the right track.  
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,631 admin
    #5
    I think you are on the right track. Finding "matching panels" to replace damaged panels and/or add capacity down the road has always been a "weakness" for solar panels... Sometimes you can find a matching panel from old stock, but they can cost 2-5x per Watt  as much as current production panels.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    edited July 9 #6

    Thnx!  I chose to add one panel to get ‘over the hump’ and excessive heat.  The upgrade will easily cool my 20’ RV in 106° weather while maintaining battery capacity (LiFePO4 24V280AH).  System size 1kW

    The off grid system currently generates 20AH in peak, summer load is about 24AH, an upgrade should produce about 34AH. 

    And I checked with REC Group to add a solar panel.  They said REC Solar N-PEAK 400 meets array tolerances and should fit the Bill 😁.


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