Using diodes to "protect" mismatched panel wattages?

2twisty2twisty Solar Expert Posts: 197 ✭✭✭
My system is a mashup of panel wattages.  Since I have purchased panels in batches, I can't get the same panel -- it seems that every 6 months they all change.  Grr.

Anyway, I have 4 strings of 3 panels.  Strings #1 & 2 are 2x230w and 1x250w panel. String 3 is 3 250w panels. String 4 is 3x295w panels.

They are in parallel into my Classic 150.

The Vmp and Voc of the 295w panels are slightly higher than that of strings 1 & 2.  When they are all in parallel, my DC amp clamp says that string #4 (295x3) is making about 11A, where strings 1 and 2 are making about 9A. And the voltage of the larger panels gets dragged down to the lower voltage.

Should I be concerned about damage to strings 1 and 2 from strings 3 and 4?  If so, could I install blocking diodes on the strings to prevent backfeeding?  If so, any recommendations?

Yes, I know I'm losing efficiency, but I can't afford to buy panels in groups larger than 3 at a time.


  • mike95490mike95490 Solar Expert Posts: 8,067 ✭✭✭✭✭
    edited August 2018 #3
    You DON'T use diodes.
    Panels in parallel need to be within 10% of each others voltage, or you are wasting power.  The higher voltage panels will do all the work, the lower voltage ones, will just be idle.

    You have to get smart with your strings.  Since you have a classic, you need to set the PV input voltage 2x higher than the battery voltage, at a minimum.

    So first forget the watts of the panels,  Look at the
    specs.   You want to match them up so series panels are within 10% of each other too.

    Try this
    A    A    A
    B    B    B
    C    C    C

    wiring is 3S3P
    wire a panel A in series with B , C   Make 3 groups , and then parallel them.  That should work better then all in parallel
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • BB.BB. Super Moderators, Administrators Posts: 28,315 admin
    As long as the Vmp-array-string voltages are within 10% -- There should be no problem (i.e, some strings have panels with Vmp~35 volts, and other strings have Vmp~36 or 38 volts... It is not ideal, but close enough that you would not measure/see the difference in day to day usage. You do not want a Vmp-30 panel/string in parallel with a Vmp~36 volts or higher (or Vmp~18 volt) strings. The larger the difference, the less efficient the array will be when connected to one MPPT controller (there is no "one optimum" Vmp-array voltage when you have strings with large variation in Vmp-string voltages).

    Diodes were used in the old days to protect panels against backfeed and shorts--But today, each panel should have a breaker (or fuse) sized per the data sheet (probably a series fuse/breaker rating of 15-20 amps depending on panel rating). This protects one string (if shorted) from being feed too much current from the other two+ strings.

    Fuses are OK, but the new style "touch safe" fuse holder cannot be "opened" under load--The fuse holders can arc and ruin the fuse block or even start a fire. Circuit breakers are nice, but usually somewhat more expensive. Breakers are great to use as switches when debugging your system.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 2twisty2twisty Solar Expert Posts: 197 ✭✭✭
    I have all my strings going into an MNPV6 with 15a breakers per string.  

    I'll have to go get the specs from all the panels so I can better discuss this.  It'll be a bit before I can do that (life, ya know)

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