Creative module wiring for expansion

wirenutwirenut Posts: 22Registered Users ✭✭
I have a cabin with 2 older 165 watt 72 cell modules on the roof. They are roughly 35v Vmp and 5A Imp. There is a cable with 2 #10 wires running from the roof to the basement where the electronics are located. I want to install more 72 cel lpanels and use the same wire so I plan to put the new mods in series. The Outback FM60 will handle the voltage conversion to the 24v battery bank. I know I don't want to put a modern 72 cell module in series with one of the old ones because it will try to push roughly 9 amps thru the 5 amp rated mod. So, what if I put the 2 old panels in parallel and then put that set in series with the 2 new mods? 
In other words, will two 5 amp mods in parallel work properly and safely with 2 more 9 amp mods in series? This should give me a total output of around 105 Vmp at about 9A Imp. I just don't want to burn up one of the older mods by trying to put too much current thru it.
Thanks for the help.


  • mcgivormcgivor Posts: 2,390Solar Expert ✭✭✭✭✭
    Can you provide full details of both old and new panels, without knowing the specs, it's not possible to offer any useful information, other than to say the values need to be close, current in the case of series, voltage in in the case of parallel.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    900W  3 × 300W No name brand Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal as a backup system. 
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergencies and welding.
  • wirenutwirenut Posts: 22Registered Users ✭✭
    The old panels are Voc 44.5, Vmp 35, Isc 5.4, Imp 4.72. The new larger panels are Voc 46.81, Vmp 37.33, Isc 9.54, Imp 8.97. It seems like the two panels in parallel with Isc 5.4 would be enough to pass the current from the other two in parallel with Isc of 9.54. Kirchhoff's current law would suggest this should work. I just don't want to burn out a panel.
    Thanks for your help
  • EstragonEstragon Posts: 3,006Registered Users ✭✭✭✭✭
    If I'm understanding the plan, you may want to check the max Voc at lowest recorded temp in your area. If it gets cold, you might risk overvoltaging the Outback.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Posts: 28,137Super Moderators, Administrators admin
    Very roughly, placing two old panels in parallel gives you the equivalent of a single new panel within 10% of the Vmp/Imp of the new panel:

    Vmp 35 volts / Imp 2x4.72=9.44 amps Imp (two up panels to make "composite" one panel)
    Vmp 37.33 volts / Imp 8.97 amps (one up panel)

    Should work...

    Depending on how cold it gets in your area, 2 to 3 "panels" in series for Vmp-array ~ 100 VDC max is usually about maximum (for cool to colder winters).

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