Is this going to work?

JesperEybye
JesperEybye Registered Users Posts: 2 ✭
in Solar Beginners Corner #1
Will 40 of these cells be enough to power a 12v off grid battery system, even on a cloudy day?

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  • mvas
    mvas Registered Users Posts: 395 ✭✭✭
    #2
    Maybe, maybe not ... it all depends upon how cloudy it is and how much power you need.

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  • JesperEybye
    JesperEybye Registered Users Posts: 2 ✭
    #3
    Cloudy like, it isn't totally dark, but the sun isn't shining directly on the panels either.
    I need 14 volt or so to charge the 12v battery

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  • Photowhit
    Photowhit Solar Expert Posts: 6,011 ✭✭✭✭✭
    #4
    JesperEybye said:
    Cloudy like, it isn't totally dark, but the sun isn't shining directly on the panels either.
    I need 14 volt or so to charge the 12v battery


    Even under room light they will likely produce 14 volts, but next to 0 amps/current.

    Voltage is NOT a measure of power, wattage is a measure of power, watt= amps x volts.

    We would want a lot more info; size and type of battery, actual loads (this isn't going to run your house)

    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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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #5
    It would have to be a pretty small battery. You might get a couple of amps at ~14v to charge it with thin cloud.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,654 admin
    #6
    Nominally, 4.1 amps @ 18 volts would require 36 cells. The "standard" 12 volt battery bank charging panel configuration.

    For a lead acid battery bank, we suggest 5% to 13% nominal rate of charge (20 Hour Rate). 5% can be OK for weekend/emergency use. 10%+ rate of charge is better for full time off grid:
    • 4.1 amps * 1/0.05 rate of charge = 82 Amp*Hour @ 12 volt battery bank maximum
    • 4.1 amps * 1/0.10 rate of charge = 41 Amp*Hour @ 12 volt battery bank nominal
    • 4.1 amps * 1/0.13 rate of charge = 31.5 Amp*Hour @ 12 volt battery bank minimum
    Your panel would be rated (36 cell) at Vmp~18 volts and Imp~41 amps. It would run a DC load on 12 volt battery bank of (assuming 4.0 hours a day minimum sun--Non-Winter time):
    • 18 volts * 4.1 amps = 74 Wmp
    • 74 watts * 0.61 DC off grid system eff * 4.0 hours of sun = 181 Watt*Hours per day
    • 181 WH per day / 12 volts = 15 AH per day
    Now--The hard part is to install those cells into a panel. My recommendation is not to bother... But if you are really interested in trying, here is one guy that went to a lot of trouble to do it (approximately) right:

    http://fieldlines.com/board/index.ph...,144982.0.html
    http://fieldlines.com/board/index.ph...,144995.0.html
    http://fieldlines.com/board/index.ph...,145004.0.html

    And here is what can happen if the panels are built "wrong":

    Panel Fire Question

    My suggestion is to buy good quality factory panels (glass front, mono or poly crystalline cells). It is very difficult to build "good" panels from raw cells--And many of the raw cells are actually factory rejects from panel manufactures.

    Panels used to be $10-$30 per Watt (10-20+ years ago). You can buy a pallet of panels today for around $1-$2 per watt (plus shipping-which may not be cheap). Spend your time & money on  robust design (first) and good quality equipment.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #7
    Today on a cloudy day, my 5Kw of pv produced about 2Kwh.   If you don't have bright direct sun, you don't get much power.

    And it looks like you have wafers, and are going to build a panel.   6 months later, you are likely to have 0 watts.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • mvas
    mvas Registered Users Posts: 395 ✭✭✭
    edited February 2017 #8
    How many Amp-Hours is this "12 volt off-grid battery system" ?

    Your plan is  ... 40 cells x 0.5 volts each = 20 Volts x 4.1 amps = 80 Watts.
    80 Watts is a very small solar panel.
    80 Watts / 15 Volts (charging) = 5 Amps into the battery with MPPT Charger.
    So, maybe you use a single 12V 100AH battery with 50% DOD max draw down.
    That is OK for LED Lights and very small loads, only.

    On a cloudy day you might generate only 2 Amps ( 32 Watts / 15 Volts )
    That is very low, but yes it is do-able.

    Q1) What size battery do you need for "off-grid" ?
    Q2) What is the Mfr & Model # of the Charge Controller ?
    Q3) What are your expected Loads ?

    You need to protect the raw cells from humidity and oxygen = severe corrosion
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