Array dimensions: 3x2 or 2x3?

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2twisty
2twisty Solar Expert Posts: 199 ✭✭✭
in Solar Beginners Corner #1
48v system. Panels are 250w. VMP is 38v.

So, I can charge with either 2 or 3 panels in series. 2 puts the input voltage just high enough to charge my 48v bank. It works. 3 puts my input voltage over 100, so I am concerned that the cc will dissipate a lot of that power as heat. But, if I have 2 in series I am concerned that partial shade would drop the voltage and cause the charging to stop.

I'm wired as 2 strings of 3 now. Looking for opinions.

Sorry for the terseness. Posting from my phone.
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  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    #2
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    Re: Array dimensions: 3x2 or 2x3?

    So those are 72 cell panels? Unusual vmp for a 250W panel.

    3 strings of 2
    Vmp=76V. Thats enough to charge 48V. The 16 odd volt margin should mean that solar mode should work fine, without going to legacy mode. Voltage drop still pretty acceptable unless cable real long. Hot weather margin should be fine.

    2 strings of 3
    Vmp= 114V, plus a bit for cold days, puts you over 120V ELV, more care needed for wiring to code.
    CC runs hotter as you say.

    3x 60 cell panels is kinda ideal for 48V systems.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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  • 2twisty
    2twisty Solar Expert Posts: 199 ✭✭✭
    #3
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    Re: Array dimensions: 3x2 or 2x3?

    Oops. Its 38VOC.

    They are 72 cell panels

    No code here where I live, so I am interested in sensible safety, and not necessarily "to code." Cold weather should not be a problem. I'm just east of El Paso.

    Sounds like I should stay with strings of 2.
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  • 2twisty
    2twisty Solar Expert Posts: 199 ✭✭✭
    #4
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    Re: Array dimensions: 3x2 or 2x3?

    OK. I'm at a computer now. Lol.

    Here's the specs on the panels

    VOC 38.11
    VMP 30.91
    ISC 8.62
    IMP 8.09

    These are for the 5 250W panels I have. My 4 230W panels have slightly lower numbers. I tried to get more of this identical panel but they weren't available anymore so I got the 250s. I know I will experience some losses because of this, but its all I can do.

    I was planning to build the array as 3x3, but I can just as easily build it as 4x2 and have a spare panel (although that's 250w I would not harvest).

    I can't afford to buy another charge controller to separate the panel types, so I'll just have to live with the losses I get, and I'm OK with that.

    Anyway, I'd like to go with 3x3 to make use of all my panels, if possible, but if it makes overwhelming sense to go with 4x2, I can do that, too.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #5
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    Re: Array dimensions: 3x2 or 2x3?
    2twisty wrote: »
    Oops. Its 38VOC.

    They are 72 cell panels
    Well we're back to something wrong, 72 cell panels should have a VMP around 35 volts VOC should be above 40
    2twisty wrote: »
    Sounds like I should stay with strings of 2.
    With a VOC of 38 likely your vmp is too low to run strings of 2, you'll need to do strings of 3.
    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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  • 2twisty
    2twisty Solar Expert Posts: 199 ✭✭✭
    #6
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    Re: Array dimensions: 3x2 or 2x3?

    Wow. I am just FULL of errors tonight. They ARE 60-cell panels. I just counted. Maybe after 15 more posts I'll get this all right. DUH.

    OK. I was pretty sure that strings of 2 was marginal for a 48V bank, so I think you've convinced me to go with my original plan of 3x3.
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  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    #7
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    Re: Array dimensions: 3x2 or 2x3?

    PWs right, 2 x 60 cells is not enough for 48V. Not even on legacy. The headroom that the classic needs varys depending on who you ask, its at the very least 6V, but 12V, and 1x "system voltage" headrooom are more common answers. And you do need to factor both hot and cold temps into that.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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