Question about adding new array

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NBDrancher
NBDrancher Registered Users Posts: 19 ✭✭
in Off Grid Solar & Battery Systems #1
There are a lot of smart folks on this forum and need some help.  I am adding 3 x 285 watt panels to my existing off-grid system and have a question concerning my C-40 Xantrex charge controller.  The current array is a 24 volt system with a 750 ah battery bank and has 4 x 150 watt/24 vdc Siemens/Shell panels connected in series.  I am in Apache County AZ.  I measured the output voltage yesterday at 10:00 am and it was 28.75 volts.  The VOC on these older panels is 43.04.  The cumulative ISC amps should be 19.2.  The C-40 has the ability to handle 125 vdc and 40 amps of continuous charge.  The panels are about 20 feet from the outdoor disconnect that feeds the C-40. 



Shell panels.



The new array is in the foreground, 3 x 285 watt panels rated at 38.4 VOC and 9.59 ISC. I have installed a MNPV6 combiner box and lightning arrestor, ready to connect.
  



My question is, should these new panels be wired in series or parallel for the best performance?  Will either scenario over-drive the charge controller?   BTW, my inverter is an Outback FX3624.  Thank for having a look and any response is greatly appreciated.
Outback VFX3524 inverter, Xantrex C-40 charge controller, Iota 24 volt charger, Siemens panels, German Solar panels, Pre-Formed Products pole mounts, Deka 6 volt batteries, 21kw Perkins/Borg Warner diesel generator.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #2
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    A C-40 Xantrex solar charge controller is a PWM type (pulse width modulated).

    More or less, that is controller that uses transistors as an On/Off switch.

    When you connect your solar array through the C-40 to the battery bank, the solar array output will be "clamped" to the battery bus voltage (around 28-29 VDC in your case, with the batteries charging).

    The "optimal" match for a solar array on a 24 volt bank with PWM controller is around 35-40 volts (Vmp-array-std-conditions).

    Your panels are Vmp~34.0 volts. A tiny bit low for "optimum" on a 24 volt Lead Acid battery bank--But it will work fine.

    The issue is all of your Vmp~34 volt panels should be connected in parallel to your C-40 controller for optimum results.

    If you put 2 or 3 panels in series, you will only get a maximum output of Vbatt~29 volts and Imp~4.44 amps per string. That means your 2 and 3 series string actually produces almost the exact same charging current as 1 panel.

    Yes, the C-40 can manage higher voltages and current--But it is just an on/off switch.

    If you want panels in series (longer distance from array to charge controller is one reason), you would need an MPPT (maximum power point tracking) type charge controller. It can take the high voltage/low current from the array and efficiently down converter to low voltage/high current for charging the battery bank.

    Think of a PWM controller like a one speed transmission--The correct gearing for one speed and one power source. Vs an automatic transmission with can match engine horsepower (RPM and Torque) to the speed and loads of the vehicle (road speed, hills, wind, weight).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #3
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    NBDrancher said:
    ... and has 4 x 150 watt/24 vdc Siemens/Shell panels connected in series.  I am in Apache County AZ.  I measured the output voltage yesterday at 10:00 am and it was 28.75 volts. 
    Okay first lets get some nomenclature correct. This is the way series and parallel works for solar panels and batteries;

    Solar panel wiring basics An intro to how to string solar panels

    So if in series the voltage should increase and the current/amperage should remain the same, in parallel, the voltage should remain the same and the current should add.

    NBDrancher said:
      I measured the output voltage yesterday at 10:00 am and it was 28.75 volts.  
    It's important to note where and how you are talking this measurement.

    If you are measuring while connected to your charge controller, then you are measuring the system voltage and NOT the panels voltage (I suspect they are all in parallel) If you are measuring while disconnected and at your charge controller, they the voltage is very low and I would suggest increasing your wire size. Though measuring each panels at the panel might be in order to ensure that each is working properly. as the panels heat up during the day, the voltage is likely to drop further.

    NBDrancher said:
    The new array is in the foreground, 3 x 285 watt panels rated at 38.4 VOC and 9.59 ISC. I have installed a MNPV6 combiner box and lightning arrestor, ready to connect.
    This is the normal range of 60 cell panels, with a VOC(voltage open circuit) of 38.4, they likely have a VMP (voltage under load) of around 30 volts which isn't high enough to use in parallel with your charge controller effectively. While you might run 2 in series, if it's within the max voltage, you will get very little extra power from 2 in series and you won't get anything from the 3rd panel.

    I think a small MPPT type charge controller might be in order.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • NBDrancher
    NBDrancher Registered Users Posts: 19 ✭✭
    #4
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     I have to apologize to BB and Photowhit.  My original array is connected in parallel as Photowhit pointed out; "I suspect they are all in parallel".  I did take the measurement at the last panel output and not disconnected from c/c.  I will take both of your comments under advisement starting with an increase in wire size. The conductors are only #12 right now.  In the near future I will probably replace the old C-40 and upgrade to a MPPT.  I like BB's power transmission simile.
    Outback VFX3524 inverter, Xantrex C-40 charge controller, Iota 24 volt charger, Siemens panels, German Solar panels, Pre-Formed Products pole mounts, Deka 6 volt batteries, 21kw Perkins/Borg Warner diesel generator.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    edited September 2020 #5
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    NBDrancher said:
      I did take the measurement at the last panel output and not disconnected from c/c.  
    Likely represents a very reasonable system voltage for 10am! You might be in Absorb already.

    You can leave your old panels connected through the PWM charge controller, You will want solar panels with similar output going through your MPPT controller. If you want to add panels later you will want to add, basically, panels with the same number of cells, so a quality MPPT charge controller, and FM80 or Midnite classic, will likely handle a series/string of 3 of your new solar panels. If they are as I suspect 60 cell panels. but if you wanted to expand them, you would add another string of 3 - 60 cell panels.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • NBDrancher
    NBDrancher Registered Users Posts: 19 ✭✭
    #6
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    Taking everyone's advise to heart I purchased an Outback Flexmate 80 to connect my new array and will leave the "Old" Siemens panels connected to the Xantrex C-40.  I am digging a hole for another TPM-3 pole mount for 3 more 285 watt panels. 
    Outback VFX3524 inverter, Xantrex C-40 charge controller, Iota 24 volt charger, Siemens panels, German Solar panels, Pre-Formed Products pole mounts, Deka 6 volt batteries, 21kw Perkins/Borg Warner diesel generator.
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