Advice Needed on acceptable string configuration(s)

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Shawn
Shawn Registered Users Posts: 5 ✭
in Advanced Solar Electric Technical Forum #1
Hello Everyone,
I just joined the forum, but I have been reading (lurking) as a guest for a few months. I have a new to me off grid cabin in Northern AZ with a really old solar system. It is currently a 12V system with a big battery bank that I inherited from the previous owner. (modified sine xantrex and 10 T105 batteries in 5 parallel strings) yeah I know hat is not a good thing. It is being -barley- changed by 6 80W shell panels... It has a Tristar PWM 45 charge controller. (probably the nicest part of the whole existing system :') ) I do have an older honda generator also...

I am a "bargain hunter" yes one of those...
I have a "new" XW 48V inverter and an XW mppt 60 charge controller. that I want to install. I will probably buy 6 more batteries to make two strings of 48V... or I may drop 2 batteries out of the main string, and just keep the old inverter, charger, and panels for my "shed" at the property. I guess it would be nice to have a 12V system around...
They batteries are new. I made the seller replace them just a month ago.

I was able to buy some -very- cheap solar panels from two individuals (who were grid tied in Phoenix), that purchased several pallets and couldn't fit all the panels they had on their roof. Both sets of panels are ET brand. I have 6 275w and 4 230w panels. The record low temp for my area is around 0... (Are charge controllers really permanently damaged if they exceed 150V? Wouldn't a 20 cent fuse be a nice idea if that is the case???)

Here are the specs for the ET panels:
Watts (STC) 275W
Watts (PTC), No
Maximum Power (Vmp) 34.95V
Maximum Current (Imp) 7.87A
Open Circuit Voltage (Voc) 44.12V
Short Circuit Current (Isc) 8.41A
Power Tolerance 0 to +5W
Module Efficiency 14.17%
Mechanical Characteristics
Cell Technology Polycrystalline
Cell Size 156×156mm
Cells Per Module 72

Watts (STC) 230W
Watts (PTC), No
Maximum Power (Vmp) 29.40V
Maximum Current (Imp) 7.82A
Open Circuit Voltage (Voc) 36.50V
Short Circuit Current (Isc) 8.30A
Power Tolerance 0 to +5W
Module Efficiency 14.14%
Mechanical Characteristics
Cell Technology Polycrystalline
Cell Size 156×156mm
Cells Per Module 60
Dimension 1640*992*50mm

I'm not 100% concerned about getting "maximum efficiency" out of every panel. This is way more watts than I have now, but would like to not buy another controller if possible. I don't want to damage anything or do anything unsafe...

I know I can't run 3 275w panels in series as that exceeds the voltage capacity of the controller on that one super cold day we might have...
I can run 3 230w in series... I was wondering about a series / parallel configuration of 2 275w's and one 230w in series and 3 banks of them in parallel. I know I will need to fuse more than 2 parallel strings. The amps are pretty close but the voltages are pretty significantly different. This would leave me with one extra 230w panel, but I am ok with that. I can keep it for another project or sell it cheap like I got it...

I guess In a "perfect world" I would run 2 275w's in series and 3 banks in parallel and 3 230w panels in series on another controller.I don't think the 230w panels make enough voltage to equalize a 48v battery bank if I run them in 2 x 2 configuration...

What should I do with the shell panels? I could drop 2 of them and run 4 of them in series on the existing PWM controller, but is that even worth the effort? I know things would be easier if I had a 24v inverter, but the deal on the inverter and charger was too good to pass up...

Thanks for any advice. And don't be afraid to tell me I don't have a clue what I'm doing, I'm just learning. I do have a engineering degree, so I can be lead down the right path technically!

Thanks for a great forum!
-Shawn
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #2
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    Re: Advice Needed on acceptable string configuration(s)

    Personally, I think I would use the 6 - 275 watt panels in 2 strings of 3 and the old panels in one string of 6 with the new Charge controller. The put the 4 230 watt panels on the old CC in 2 strings of 2.

    While 2 strings of 2 will not reach equalizing voltage or help any after the batteries near the absorption level, they will help when your system is seriously low in the bulk stages. Indeed once your batteries reach absorption they will reduce the amount of current they will accept.

    The MPPT CC can be damaged by too high a voltage. I thought the Xantrex was similar to the Midnite CCs in that it can take a bit more voltage with out harming it,(I checked and I'm wrong! pretty serious about not exceeding the 150v level!) but I would be sure to read up on this! MPPT are sensitive to too high a voltage!

    We would need more information about your energy use to know weather it would be more practical to remove 2 batteries or add another string...
    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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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #3
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    Re: Advice Needed on acceptable string configuration(s)

    So since the strings of 3 will not work, lets try 3 strings of 2 and 2 strings of 3 of the Shell 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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  • Shawn
    Shawn Registered Users Posts: 5 ✭
    #4
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    Re: Advice Needed on acceptable string configuration(s)

    I know that for the XW MPPT 60 140V is cutoff and 150V is "Bad". Using the midnite solar calculator and using .8 for the 230w panel in series with the two 275w panels yields this... (See the attached image.) I assume I can use .8 for the smaller panel as that is the percent difference in the voltage... The amperage is pretty similar. I guess I wondering what it will do to the MPPT programming?

    I have read many posts here people want to combine 1 very small panels with large strings. I'm just looking for a how bad will be vs a two charger setup. I guess I could try it, but I was hoping to get a technical answer. Will the addition of the 230w panel reduce both the other 275w panels by 45w/ea? Is that worst case, or is there another issue that I'm not aware of? I may be able to live with losing 270w to gain 690w and save $500.
    -S
    -Shawn
    Attachment not found.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #5
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    Re: Advice Needed on acceptable string configuration(s)

    The problem is the 230 watt panels do not have high enough voltage to charge the 48 volt battery bank in pairs. I believe Bill suggests that you can hook them up and they will not effect the charging from the other higher voltage panels, but since you have the other CC, I just am suggesting take it out of the equation.

    Typically your looking for the voltage to be within 5-10% for parallel panels or strings on a MPPT charge controller.

    BTW - your attachment doesn't work.
    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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  • ramloui
    ramloui Solar Expert Posts: 109 ✭✭
    #6
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    Re: Advice Needed on acceptable string configuration(s)

    What if you put 2 of the 275W panels and 1 of the 230W in a string? Voc for the string would be 124.74V at 8.3A. Put 3 of those strings in parrallel and you get a total of 124V at 24.9A to the CC and you would be left with 1 x 230W panel to use elsewhere or sell. Vmp of the string would be 99.3V which is just about perfect for CC efficiency, right? Would 124.74V be too high a Voc for the coldest day? If it is, what are the probabilities that this record cold T will happen during the lifetime of the array?

    Just a thought...
    Cheers!
    Off-grid cabin in northern Quebec: 6 x 250 W Conergy panels, FM80, 4 x 6V CR430 in series (24V nominal), Magnum MS4024-PAE
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  • Shawn
    Shawn Registered Users Posts: 5 ✭
    #7
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    Re: Advice Needed on acceptable string configuration(s)

    Yes that is what I was asking initially. Can I do that? Is it a valid configuration.
    It is good to -40 without damage and 0 fahrenheit before shutdown. I'll try to upload the attachment again.
    Thanks for responding!
    -ShawnAttachment not found.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #8
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    Re: Advice Needed on acceptable string configuration(s)

    I think that will work, since the ISC is very close. I guess you'll have to set one aside. I hadn't run across this situation before, but the ISC is about the same so the amps through the string should remain the same as the lowest panel and the voltage should add...
    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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