Don't know much

rbtrrerrbtrrer Registered Users Posts: 22
Given, 2/185w panels in series, ~200ft 8ga wire to mppt cc, 400ah @ 24v.

Even at +10F (freqently) to -30F with reflection from snow, seems defecient to me.

Would appreciate elaberations from the gurus. Thanx

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Don't know much

    Well 370 Watts of panel isn't going to do much for charging 400 Amp hours of 24 Volt battery:

    370 Watts @ 77% typical efficiency = 285 Watts / 24 Volts = peak current potential of 11.875 Amps, or 2.9% charge rate. That's well under the recommended 5% net minimum. It would keep the batteries up provided there was no usage, but don't expect it to charge them from even 25% DOD.

    As for the wiring, even 8 AWG at 200 feet on 48 Volt nominal and about 8 Amps current will suffer significant Voltage drop. Rough calculations show it will be more than the 3% usually recommended as a maximum for loss. Looks like closer to 4% by my estimation.

    Never mind about "bonus power" from cold temps or snow reflection; you need to design a system so that it operates well under average conditions and won't fall outside maximum/minimum limits at the extremes (cold temps causing too high a Voc, hot temps causing too low Vmp).
  • rbtrrerrbtrrer Registered Users Posts: 22
    Re: Don't know much

    Thanx for reply Coot. This is some friends system. The 200ft is required for clear few of sky.

    Would you please guestamate approx. panel & wire size adaquate to this situation?
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Don't know much

    i'm afraid coot is right that even if you didn't have the v drop losses that you are severely undercharging those batteries. ideally, a 10% rate is good to have and that would be seen as 40a, but 5% is minimum and would be 20a delivered to the batteries after all losses and deratings. some batteries won't even like the 5% minimum we recommend and depends on the battery manufacturer and battery type.

    i'd recommend doubling the amount of pv you have. you would need to upgrade the wiring too and try to get it below at least 3% if you can't get it to 2% or lower. now we could also be misinterpreting that 200ft if that is the total wire used and the approximate distance to the pvs is about 100ft. in that case you are fine in your v drop losses, but any expansion in pv will up the current on the wire and the v drop loss unless you place all of the pvs in series. placing them all in series may or may not be possible depending on the specs of the pvs and the controller you are using.
  • rbtrrerrbtrrer Registered Users Posts: 22
    Re: Don't know much

    200ft one way. they do have Xantrax 60a mppt cc.

    What ga wire would suffice?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Don't know much

    Depending on what the panel and charge controller specs are, it may be possible to increase power by upping the array Voltage which will negate some of the V-drop problem. Unfortunately you can run into trouble with a cold Voc exceeding the controller's Vmax in fairly easily. To wit:

    185 Watt panels with Voc around 43 normally will climb to perhaps 56 under very cold conditions. Most MPPT controllers have a maximum input Voltage (for Voc) of 150. So you see that three such panels in series can exceed that in cold temperatures. The Midnight Classic being the best-known exception with its "hyper-Voc" feature.

    For that much battery bank I'd try for an array of about 1200 Watts:
    40 Amps peak charge current * 24 Volts = 960 Watts less 77% efficiency factor = 1246 Watt array.
    1246 Watts * 4 hours equivalent good sun = 4984 Watt hours * 50% overall efficiency = 2492 Watt hours daily, which is about spot-on for 25% DOD of a 400 Amp hour 24 Volt battery bank (100 Amp hours * 24 Volts = 2400 Watt hours). Yes, there is some "fudge factor" in that (like not converting the DC Watt hours to AC in the second formula).

    As for the wire size/V-drop problem ... you'd need 6 AWG to overcome the existing drop. If you double the panels in parallel it will become a need for 2 AWG. This is definitely a case where investigating the possibilities of a higher Voltage array & controller would be worthwhile. Three in series * two strings would give 1110 Watts of panel and about 8% charging rate. You'd still need 6 AWG to handle the current & keep the V-drop minimal, and a Midnight Classic 150 to deal with the cold Voc problem.

    Just an example; not a precise design.
  • rbtrrerrbtrrer Registered Users Posts: 22
    Re: Don't know much

    Thanks again guys. I had hopes my sense of things were not as correct as it turns out.

    I've recommended this forum, Even if I had a legitamate opinion on the subject I would not care to be the one to deliver it.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Don't know much

    ok, not knowing which 185w pvs they have i made the assumption of using kyocera kc185s as the example in my calculations. for 2 in series this gives a 47.2vmp, 59voc, at a current of 7.84a. with the present arrangement i get about 5 and 1/4% on the v drop. #6 would put that at about 3.3%. #4 would put that at 2.07%. if you parallel another string of 2 kc185s this doubles the current and doubles the v drop percentage to about 10.47% with the present #8 wire. if all 4 pvs are placed in series the current stays the same and thus the v drop percents will look better than they did with 2 of them in series because of the higher voltage present putting the v drop at 2.62% with the same #8. the voc of 4 of the kc185s would be 118v and should be fine as far as i know without doing any calculations on the temp coefficients.

    so there's the easy solution to it all is by putting 2 more pvs in series if specs are for kc185s.
  • rbtrrerrbtrrer Registered Users Posts: 22
    Re: Don't know much

    Niel-I don't know the specs on the panels.

    Am I uderstanig correctly, if the panels are as you describe, adding 2 more in series, paralell to existing panels in series, would be adequat with the existing 8ga?
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Don't know much

    no to series/parallel.
    placing all 4 pvs in series with each other will do it. a series/parallel setup you don't want as that would double the current and up the v drop percentage on the pv wire run. with all 4 in series this ups the voltage (94.4vmp) rather than the current (stays the same at 7.84a) and that will actually cut the v drop % in half over what it is presently all with the same #8 wire. add to that the downconversion the mppt60 does will transform the output to 24v with twice the current which those batteries desperately need to get a better charge rate. if they aren't kc185s then tell us what ones they have and a recheck can then be made that it will be fine to use in a totally series configuration.

    a critical area to watch would be the v drop from the mppt cc to the batteries as any drops there could fool the cc into thinking the batteries are charged when they aren't. this is also a present possibility as well that could be contributing to the batteries not getting up to snuff. find out what the total wire length is from the cc to the batteries and what gauge wire is being used and i can make some better determinations on it.
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