Series Strings of different panels

CaribooBillCaribooBill Registered Users Posts: 3
I have an off-grid solar power system that is 24V. The panels do not say they are 24V panels, but list a voltage much lower, around 17.7V. There are 2 panels in series, one is an 80W, the other is 140W. There are 2 strings of these going into a combiner box. The 80W panel shows an Ioc of 4.82A, the 140W panel shows an Ioc of 8.68A. I have just been reading that this is a no-no. Should I re-string the two 80W panels in series and the two 140W panels into a separate series string into the combiner box?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    You are correct... You should match the Imp of the series panels within 10% or better. Put the pair of 80 watt panels in series, put the pair of 140 watt in series. Then combine the two strings in parallel (Vmp~35-36 volts for the array).

    In theory, you may need a series fuse for the 80 Watt panel string--See if you can find a series fuse rating for those panels (may be around 8-10 amps). In practice, you may not need the series protection fuse (the fuse protects the 80 watt wiring/panels if there is short, and the 140 Watt panel string feeds current into the short circuit).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Those are what is refereed to as 12 V nominal panels, 2 in series will give you a nominal voltage for 24 V batteries
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
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    West Chilcotin, BC, Canada
  • CaribooBillCaribooBill Registered Users Posts: 3
    Thanks for the feedback. My combiner box has 15A circuit breakers that each positive lead is connected to. Do I need a fuse as well? Your reply, BB, seemed to imply that the fuse was optional. I have just replaced my batteries with new ones. The old and new ones were/are 4 - 6V - 220A - AGM by Discover. The old ones lasted about 3.5 years, much less than what I was told - 5 to 7 years. I have 2 generators - a 6500W cheapo that puts out 120 and 240V that is very loud. I also have a Honda 2000i that is very much quieter and a lot more expensive, but cheaper to run. I have noticed that when the charge cycle gets to float, there is hardly any current. Am I doing the batteries any harm by using the Honda to get to Full Charge during the 4 hour float stage? I really appreciate your feedback and Westbranch's clarification on the panel voltage question. If I understand it correctly, any panel can be used in any system by stringing enough panels together in series to get a high enough voltage to charge the batteries. Is the ratio of total panel voltage required to nominal system voltage about 1.5?
  • EstragonEstragon Registered Users Posts: 4,495 ✭✭✭✭✭
    The ratio of panel to nominal voltage depends on the controller. Some can take quite high voltages.

    AGM can be harder to maintain for long life, and need to have proper temperature corrected charging voltages. I wouldn't use them myself unless there was a compelling reason in the application to do so.

    I normally use the generator just for the bulk and early absorb phase. Depending on the charger you use, continuing to charge through absorb to float (about 2 hrs for mine) with the generator won't hurt, but isn't needed every day. Even on cloudy days, my PV will often produce enough at low absorb currents to finish. With AGMs, you would want to make sure the AC charger is outputting the recommended DC charging voltage in absorb, ideally with a remote temperature sensor for proper compensated voltage.

    Float is normally reached around 1-2% of capacity if absorb is ended based on current, so around 2-3a in your case. The current will drop to nothing while the battery voltage goes from absorb voltage (eg 14.4) to float (eg 13.1). Once at float, with no loads, it will take a small amount of current to keep it there.
    Off-grid.  
    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. Super Moderators, Administrators Posts: 32,007 admin
    edited May 2017 #6
    For one or two matched strings of solar panels, normally you did not need series fuse per string. For 3 or more parallel strings, just on the cusp of needing a series fuse pet string.

    You have 2 types of panels, with one type 1/2 the current rating of the other. The smaller panels csrry less current and may need a fuse to protect against a short circuit fed by the larger panels. Ideally, you need to find the series fuse rating of the smaller panels. However, you may not find a rating. Smaller/older panels tended to not list any fuse rating.

    Without a mfg fuse rating, it is a bit of a guess.

    Practically speaking, if you do not add any more parallel strings (just the two you have), you do not need a series fuse.

    - Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CaribooBillCaribooBill Registered Users Posts: 3
    There are references to FAQ's in some of the general posts I have been reading. Are they on this site - I can't seem to find them.
    Estragon - your description of the charging cycle using a generator seems to be what is happening with my system, so that is reassuring. BB - thanks for the response but I wonder if my circuit breakers actually do anything. I have decided to increase my panels to 3 @ 260W in series and maybe use the 80W and 140W for something else. My Xantrex SCC is an XW-MPPT60-150 and seems to have lots of reserve for the higher voltage put out by these new panels - a Voc of 38V each. Any concerns if I do this???? My inverter is a Magnum MS4024PAE with a BMK and ME-RC remote.
  • EstragonEstragon Registered Users Posts: 4,495 ✭✭✭✭✭
    There's a bit of a trade off in how you arrange strings. Depending on temperature, 3 in series would be ~90-100v. The trade off is the charge controller will be slightly less efficient than at ~60v, but that's offset by more wire losses for a given wattage and wire size. Going longer distances, higher voltage is preferred to keep wire costs and losses down. If your batteries are right under the panels, strings of 2 might be slightly better.
    Off-grid.  
    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
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