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BKing49's avatar
BKing49
Explorer
Apr 26, 2014

Solar Design Review / Help

Hi all! I'm putting together a solar charging design for my older Class A RV. I've upgraded my 2 12V Marine Deep Cycle batteries to 4 Lifeline 6V Golf Cart AGM Batteries. Got them all wired up with 2/0 SGX cable. Made custom length battery cables, crimped and soldered w/copper tinned lugs. Just got my new Blue Sky 3024iL MPPT Controller. Moved the battery bank from under the chassis in a metal battery box to under the kitchen bunk seat inside the RV. Figured no venting required as the new batteries are AGM. There is room to put the charge controller in with the batteries. Yesterday I got 2 Renogy Monocrystaline PV's 250W @ 24V. Intend on wiring them in parallel to make the 500W total. Comes with 10AWG whips that I'll cut to length to get into a homemade PVC rooftop combiner box. It'll be 8AWG cable down to the charge controller. Bought a e2 fuse holder w/30Amp AGU fuse to go in the positive leg to the charge controller. Thought I'd use 6AWG on the output side of the charge controller through another e2 fuse holder with 25Amp AGU fuse to the input side of the battery bank. The ouput side of the battery bank (2/0 cable) out through the floor of the RV down into the old battery can to a e2 150Amp fuse and onto positive and negative isolation posts where I'll connect to the existing 1/0 SGX cable that runs about 15' to the power center.

That's basically it for now, but before I connect everything up, I'd like some assurances that wire gauge selection and fuse sizing is appropriate. Also, on the net I've been reading about proper safety deration of 1.56% and 1.23% of current before and after the charge controller is correct. If that's the case the Voc from the PV's doubled (because they're in parallel) plus the safety factor may put me over the max 30amp input to the charge controller. Not sure I did that right.

PV Specs
Vmp=31.64V
Imp=7.90A
Voc=37.78V
Isc=8.35A

Charge Controller 40A@12V 30A@24V

So I'm 24V from PV's to Charge Controller and 12V to batteries.

Did I do this right?

Thanks for your comments in advance.

Bruce
  • westend's avatar
    westend
    Explorer
    Apr 26, 2014
    You should be fine with that setup. If you up your combiner-> controller and controller-> battery wire size, you'll get a bit more power. I ran 4 AWG in both those locations.
  • pianotuna's avatar
    pianotuna
    Nomad III
    Apr 26, 2014
    Hi,

    I would have gone with a 45 amp controller.

    You don't mention a temperature probe--where I live that is essential.

    Tristar MPPT 45 with temperature probe would be better.
  • BoonHauler's avatar
    BoonHauler
    Explorer
    Apr 27, 2014
    If you cut the MC4 connectors off the panel wires you might void the panel warranty. Other than that all else looks to be okay in my opinion other than what Don has too say.

    Just my 2 cents.......
  • time2roll's avatar
    time2roll
    Nomad
    Apr 27, 2014
    Just get some MC4 "Y" connectors and skip the combiner box. Then get some MC4 cable with #8 wire to pull down to the controller. Plug and play.

    No fuse needed on the panel side. All wire exceeds the panel supply potential. A dead short will not even warm the wire and no harm to the panels. Although a switch to cut power for service could be helpful.

    AGM is not exempt from venting requirements to meet RVIA and NEC standards. But I would do same as you.

    25 amp fuse to the battery is small. Need 50 amp fuse on the positive close to the battery. 40a x 1.2 = 48 ~50a. I recommend a 50a Bussmann high-amp breaker that can be switched off for service. #6 wire is fine for that short distance.

    40a controller is fine IMO.

    That is my two cents. If it seems odd maybe I misread something.
  • AlbertaNewbie's avatar
    AlbertaNewbie
    Explorer
    Apr 27, 2014
    With the 250w panels flat on the roof you will never see Imp rating, figure 15% loss whennot aimed at sun. So 7.9x2 for 24 to 12v conversion is 15.8a x 2 for 2 panels = 31.6amps less 15% = 26.86amps best case scenario. You'll be fine with 30amp controller. Also running the panels in parallel will not double the voc, series will and may be a better option if you sticking with the 8awg cable to the cc. Id go 6awg to the controller from the combiner and 4awg to the batt for max potential.
  • BKing49's avatar
    BKing49
    Explorer
    Apr 27, 2014
    Thanks all for the responses. One response indicated I should up the wire size to the batter to maybe 4AWG. What kind of wire should I use? A lightweight battery jumper cable? Or Tray Cable wire? As far as the fuse from the charge controller to the batteries, I will up the fuse size to 50 although the charge controller only puts out 30A I believe. Remember the charge controller is receiving 24V and regulating out 12V to the batteries so in that scenario the manufacturers literature says max 540W PV's. All this is so confusing. I don't want to start a fire. Summers here are bright and hot. Even if the PV's are mounted flat, I'm concerned that in the mid summer sun, I will see high amps coming to the charge controller even though the PV's are only rated 8.35Amps open circuit.

    Thanks again for your comments.

    Bruce
  • AlbertaNewbie's avatar
    AlbertaNewbie
    Explorer
    Apr 27, 2014
    BKing49 wrote:
    Thanks all for the responses. One response indicated I should up the wire size to the batter to maybe 4AWG. What kind of wire should I use? A lightweight battery jumper cable? Or Tray Cable wire? As far as the fuse from the charge controller to the batteries, I will up the fuse size to 50 although the charge controller only puts out 30A I believe. Remember the charge controller is receiving 24V and regulating out 12V to the batteries so in that scenario the manufacturers literature says max 540W PV's. All this is so confusing. I don't want to start a fire. Summers here are bright and hot. Even if the PV's are mounted flat, I'm concerned that in the mid summer sun, I will see high amps coming to the charge controller even though the PV's are only rated 8.35Amps open circuit.

    Thanks again for your comments.

    Bruce


    Tray cable wire 4 awg from cc to batt, 6 awg from combiner to cc, 8 or 10 awg from each panel go combiner. Also remeber that as heat goes up solar output goes down, should be in the panel literature with a nice graph.
  • time2roll's avatar
    time2roll
    Nomad
    Apr 27, 2014
    You posted controller specs that indicate 40 amps max charging 12v battery.

    500w / 13V = 38 amps

    Probably never see 40 amps. Probably closer to 30 to 35 amp max charging current. The 50 amp fuse (controller to battery) is to protect the wire from power coming out of the battery during a fault.

    I generally buy my wire & lugs here when possible:
    http://www.genuinedealz.com/marine-wire/marine-battery-cable?size=57

    Don't try to mount the panels perfectly flat as they will shed water and dust better with a slight tilt.

    MC4 8awg cable on ebay

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