Solar charging series or parallel for MPPT to 12v battery

gqrootgqroot Registered Users Posts: 22
Hi folks,

I am building a off-grid system to power up 12 watt device running 24 hrs. The build of materials below. I am wondering if I should get a pair of 12v 100 watt panel in series or in parallel to charge my 12v battery. I get about 2 hours of sun and trying to spec it for 2 days without the sun. Is it better to have the solar panel in series or in parallel when the cable run is 25 feet?

1x MorningStar SunSaver SS-MPPT-15L Charge Controller
1x Concorde PVX-340T Sun Xtender Solar Battery 12V
2x 12v 100w Solar Panel

Thanks in advance!

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,005 admin
    Ignoring Solar for the moment...

    [note: I messed up here--25 feet and 12 watts--See following post for corrections]

    That battery seems awfully small for your application (12 volt @ 34 Amp*Hours??):
    • 12 volts * 34 AH * 1/25 watt load = 16.3 hours until "dead"
    If you want to go 2 days without sun and 50% maximum discharge (for longer battery cycle life):
    • 25 watts * 24 hours per day * 1/12 volt battery bank * 2 days * 1/0.50 maximum discharge = 200 Amp*Hour @ 12 volt battery bank
    You may want to look at using 2x 6 volt @ ~200 AH golf cart batteries--At least for the first try. They may be a lot cheaper and easier to monitor (can use a hydrometer, add distilled water once a month). Once you get the system stable and working the way you want--Eventually replace with AGM (more expensive, less maintenance, perhaps a bit shorter cycle life).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gqrootgqroot Registered Users Posts: 22
    Thanks Bill for getting back. I have to use a sealed battery because I will be mounting the solar panel and battery box rig on top of a light pole that is approximately 90 feet high. The structural engineer may have some reservations with me putting my rig that is over 100 lbs. So far I calculated 85 lbs. I guess my other option is to put have the battery box on the bottom of the light pole. The light pole won't have 120v power for 3 years.
  • BB.BB. Super Moderators, Administrators Posts: 32,005 admin
    I would suggest placing all serviceable components at the base of the pole--Unless you have easy/cheap access to a lift.

    Of course, that gets security questions (if a problem). Possibly place the electrical box at a lower height (use a ladder).

    And "Oops...". I got my numbers mixed up, you are running 12 Watts, not 25 watts--Mixed the numbers up in my earlier post:
    • 12 volts * 34 AH * 1/12 watt load = 34 hours until "dead"
    • 12 watts * 24 hours per day * 1/12 volt battery bank * 2 days * 1/0.50 maximum discharge = 96 Amp*Hour @ 12 volt battery bank
    Sorry.
    • 12 watts * 24 hours per day * 1/0.77 charge controller+panel derating * 1/0.90 AGM batt eff * 1/2 hours of sun per day = 208 Watt array minimum
    • 96 AH battery bank * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 181 Watt array recommended minimum size (10% rate of charge)
    So, a pair of ~100 Watt panels would appear to fit the minimum requirements.

    You started this as a wiring length question--Solar panels mounted on pole? And yes, I probably would use the two solar panels in series to allow a smaller awg wire. Using a generic voltage drop calculator:
    • 2 * 100 Watt panels * 1/35 volts Vmp-array = ~5.71 amps Imp
    http://www.calculator.net/voltage-drop-calculator.html for 3% to ~1% maximum drop
    • 0.03 maximum recommended voltage drop * 35 volts Vmp-array = 1.05 volt maximum drop
    Using voltage drop calculator with 25 feet one way run and 5.71 amps and 1.05 maximum voltage drop:

    14 AWG wire:
    Voltage drop: 0.72
    Voltage drop percentage: 2.06%
    Voltage at the end: 34.28

    10 AWG wire:
    Voltage drop: 0.29
    Voltage drop percentage: 0.83%
    Voltage at the end: 34.71

    So, I would suggest around 14-10 AWG wire as being optimum for this installation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • gqrootgqroot Registered Users Posts: 22
    Thanks BIll for your help! To squeeze as much volts from the panel and not lose it too much is to use as you suggested 10 AWG. Thanks again!
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