DIY solar panel

Discussion in 'The Projects Forum' started by oolesh, Jul 18, 2012.

  1. oolesh

    Thread Starter New Member

    Apr 21, 2012
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    hello i have a question i have 40 3" x 3" solar cells 0.5V x 1.8A can i charge a small 12v battery how can i connect it please HELP ME
     
  2. BMorse

    Senior Member

    Sep 26, 2009
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    234
    you will need more than a "connection" between the solar panels and the battery, you will need some kind of charge regulator circuit in between....... what is the rating of the "small 12 volt battery"??
     
  3. oolesh

    Thread Starter New Member

    Apr 21, 2012
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    7ah how to connect it in series or parallel i got on ebay the seller told me it's 30 watts i have a chager controller
     
  4. BMorse

    Senior Member

    Sep 26, 2009
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    Sealed lead acid (SLA) or gel cell??

    Most SLA batteries require atleast 14.5 volts for charging, so wire your panels to produce atleast 14.5......
     
    Last edited: Jul 18, 2012
  5. wayneh

    Expert

    Sep 9, 2010
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    1.8A from a 9 square inch panel? I'm skeptical. Are you sure it's not 0.18A?

    A rule of thumb is that the open-circuit panel voltage needs to be about twice the battery voltage. All 40 cells in series should give you 20v, which is about right, maybe a bit low, for a 12V battery. You'll need a blocking diode in series to prevent reverse current discharge of the battery into the panel. I see no sense in any parallel wiring of the cells.

    The remaining concern is that you don't overcharge your battery. This depends on the real current still flowing when your charged battery is in series with all those cells, and on the specs of the battery. My hunch is that you'll be fine without a controller, but don't rely on my hunch. Do the experiment and/or read the specs carefully.
     
  6. BMorse

    Senior Member

    Sep 26, 2009
    2,675
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    You are most likely right, didn't really pay attention to the decimal placement, ;).

    And also with the wiring all of them to get 20V, since the battery will draw the output down to the battery voltage level anyway....
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,175
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    30 W seems about right, 20 V OC gives about 17 V operating at peak power @ 1.8 A. If the cells are not already wired in series, an array 8 cells in a collum X 5 cells wide, all in series will make a nice size panel, 3' X 2'. What are the specs on your charge controller??
     
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  8. wayneh

    Expert

    Sep 9, 2010
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    You're right. It's 40 x 9 = 360 sq. in, or 0.23 sq. meters. A typical panel can make 150W/m^2, so that's about right. Guess I'm just used to higher voltage and lower current cells.

    The OP will likely need a charge controller. Current into a 12V battery at full charge could be an amp or more - too much for even a car battery.
     
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  9. oolesh

    Thread Starter New Member

    Apr 21, 2012
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    yes i think it's 1.8a check this http://www.ebay.co.uk/itm/180926389352?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 and the seller say about 30 watts
     
  10. DMahalko

    Senior Member

    Oct 5, 2008
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    You need at minimum a blocking diode with the solar cells and battery. When cells are in the shade they actually consume energy and will discharge the battery. The diode stops the discharge, but it has a voltage drop of about 1 to 1.5 volts and this drop consumes the output of 2-3 cells in series.



    If all the cells are all wired in series and half the panel is in the shade, each shaded cell will consume the energy of one or more the cells in the sun. This happens regardless of if you have a blocking diode or not.

    It is vaguely possible to improve panel efficiency by wiring them as series-parallel strips, with a blocking diode for each series string. This way if one string is shaded at least the others can still produce power. But you still have greater losses due to voltage drop on each series string from each additional diode, and your total voltage for all strings in parallel is lower.



    Finally, angle of the light striking the cell surface matters. Silicon has a very high refractive index, and light only penetrates at 100% within an angle of about 15-20 degrees from perpendicular on the face of each cell. If you exceed the "critical angle", the light intensity drops off until it just bounces off like a mirrored surface, and doesn't do anything.

    So, a panel in "full sun" all day may not actually do any real charging except for a few hours when the sun is directly overhead, and slightly before and afterward. This is why people use "solar trackers" with solar panels.

    (The cell design type of monocrystalline or polycrystalline may affect the incidence angle, but I haven't seen any info about that yet.)



    Encapsulation in plastic helps to diffuse the light so it can penetrate the cell at a higher sideways angle. Plastic also helps to protect the cell from environmental damage like objects striking it, since solar panels are basically purified sheets of very thin stone and can be brittle.
     
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  11. wayneh

    Expert

    Sep 9, 2010
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    Yes, I believe it now. You will want a charge controller of some kind, since that current can overcharge even a large 12V battery.

    The simplest circuit for DIY is a constant voltage regulator set to about 13.8V (depends on temperature, and your battery). The LM317 is often used for this purpose and with a heat sink will likely work here. I think you can go to 1.5A with that IC (again, with a big heat sink), and your panel will be below that level, maybe less than 1A, when charging a battery. The 1.8A would only happen during a short. Consider a 1.5A fuse in line with the battery.

    Other experts here can likely suggest the right choice of IC and how to build a robust solution.
     
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