24 120W panels for 48v battery bank - 6 strings of 4?

Dan_
Dan_ Solar Expert Posts: 61 ✭✭✭✭
in Solar Beginners Corner #1
I have 24 KC120 Kyocera panels and a 48v battery bank. Seems like the max I could string together for my 150v charge controller is 6 but Id gain 2-3% more efficiency if the voltage was closer to 60v (which seems like a string of 4). Panel specs attached. Just wanted to confirm that 6 strings of 4 would be best. Will be pole mount with only a short cable run so very little voltage drop.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #2
    Re: 24 120W panels for 48v battery bank - 6 strings of 4?

    The closer the PV string voltage is to the battery voltage the better CC efficiency you get. But at 67.6V PV you are close to the minimum voltage for charging a FLA battery. what does your batt manufacturer require? Also it is probably too low for an EQ.
    hth
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
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    West Chilcotin, BC, Canada
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: 24 120W panels for 48v battery bank - 6 strings of 4?

    What charge controller do you have and how cold does it get at your location?

    The 4X Vmp is low for a 48 Volt system: 67.6 Volts. With panel heating and wire loss that could be problematic for Summer charging.

    By the same token 6X Voc would be 129 Volts and if the temps go cold it could exceed the V max in on anything but a Classic.

    A string of five of these panels would be best for a 48 Volt system, but with twenty-four total you'd either have to scrounge one more or forgo 480 Watts. That would make the array twenty panels and 2400 Watts with an output of about 38 Amps @ 48 Volts. More than enough if your battery bank is 208 Amp hours @ 48 Volts as per your signature.
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  • Dan_
    Dan_ Solar Expert Posts: 61 ✭✭✭✭
    #4
    Re: 24 120W panels for 48v battery bank - 6 strings of 4?

    Thanks to you both
    Charge voltage is 59.2 and eq is 62.4
    Daytime winter low temps are about 35-40f
    CC is Outback FM60
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: 24 120W panels for 48v battery bank - 6 strings of 4?

    Well the FM60 will shut down around 140 Volts input. So six panels in series could be a problem even with low temps around 35. I wouldn't want to risk it.

    This leaves either string of four or string of five. Four gives you Vmp around 67, which with a need for EQ of about 62 ... well it's pretty easy to lose 5 Volts in the system so that too is cutting it close (albeit in the other direction).

    So what about the five panels in series option? Any reason why you need more than 38 Amps peak current? Four strings of five goes nicely with a four string combiner and gives you four spare panels.
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  • Dan_
    Dan_ Solar Expert Posts: 61 ✭✭✭✭
    #6
    Re: 24 120W panels for 48v battery bank - 6 strings of 4?

    No reason. Bought the panels as a lot. Could sell some or keep as extra. Pole mount racking I have is for the 24 though (12 on each pole)
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: 24 120W panels for 48v battery bank - 6 strings of 4?

    One caution: at potentially 38 Amps the charge rate for 208 Amp hours of battery will be around 18% which is a bit on the high side. You may want to program a limit on the current to keep it below 15% (roughly 30 Amps). The extra available panel will provide a bit more current on less-than-perfect days.
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