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Thread: Open circuit voltage

  1. #1
    Junior Member
    Join Date
    Dec 2009
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    Default Open circuit voltage

    Hi,
    I bought some 1.75W/0.5V cells from ebay. Aim is to make small (proto) off-grid system for next summer. Total power would be around 100-150 W. I am also designing mppt charge controller for this, but as it happens I dont have any earlier experience with PV systems.

    For this reason I need to know some basics from PV. How much voltage fluctuates as weather changes. What should be the oc voltage be in 12V system? I understood that PV acts like current source where amperage stays the same while voltage travels. Is this so?
    I
    Last edited by jynkky; 12-15-2009 at 02:44 PM.

  2. #2
    Solar Fanatic
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    Dec 2009
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    Manabi, Ecuador
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    Default

    The standart for 3x6 cells is 36 cells/panel which generates you about 60W.

    I'm using a sandwich glass made panel, you can see them in my thread "my tiny proyect in Ecuador" in the DIY section.

    Depending where you are located they have between 18 to 21V without load.

    The watts/daily depend on the intensity of sun and your isolation factor for your zone.

    [QUOTE=jynkky;5929]Hi,
    I bought some 1.75W/0.5V cells from ebay. Aim is to make small (proto) off-grid system for next summer. Total power would be around 100-150 W. I am also designing mppt charge controller for this, but as it happens I dont have any earlier experience with PV systems.

    For this reason I need to know some basics from PV. How much voltage fluctuates as weather changes. What should be the oc voltage be in 12V system? I understood that PV acts like current source where amperage stays the same while voltage travels. Is this so?
    I

  3. #3
    Moderator
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    May 2009
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    off grid retirement in Northern Calif
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    7,218

    Default

    Quote Originally Posted by jynkky View Post
    Hi,.... What should be the oc voltage be in 12V system?


    I understood that PV acts like current source where amperage stays the same while voltage travels.
    a 12V battery needs about 15V to get fully charged. The charge controller consumes 2 volts, of internal diode drops. Allow another volt for losses on a hot day.
    So your panel Volt Max Power should be around 18 or 19V Open ckt voltage around 21V


    PV is crrent sounce, at a constant voltage. So when there is enough light in the AM to start the panel, you are at 20V, 5mA Brighter light = 19V, 3A Noon sun = 18.7v 5A As panels get hot, their voltage begins to sag. Amps vary according to intensity.
    Since the dawn of time it has been mankind's dream to blot out the sun.
    Montgomery Burns

    "Always listen to experts. They'll tell you what can't be done and why. Then do it."

    spreadsheet based voltage drop calculator:
    http://www.solar-guppy.com/download/...calculator.zip
    http://www.solarpaneltalk.com/showth...oss-calculator

    http://www.mike-burgess.org/PVinfo_2.html

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,
    battery lugs http://tinyurl.com/LMR-BigLug
    Setting up batteries http://tinyurl.com/LMR-NiFe

    gear :
    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 || || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

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