What size solar panel?

Discussion in 'Power Electronics' started by John Berry, Mar 21, 2017.

  1. John Berry

    Thread Starter Member

    May 17, 2016
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    Hello, I am having trouble trying to find out what size solar panel will be ok for my 12v 3.2Ah battery and my 12v 11Ah battery.
    ( if there is an equation I cant find it )
    I am wanting a solar panel that would cope with both sizes.
    The batteries will be used separately, I know that they cannot be charged together.
    I was also thinking about a 20 or 30 Amp controller, would that be enough or maybe even a 10Amp ? Thank you very much. John :)
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Assuming they are lead acid batteries they are usually charged at a current no more than C/10. That would be .32 amps for the small one. This may make it real slow for the big one so you might want a switchable charge controller.
    http://www.powerstream.com/SLA.htm#Fast
     
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  3. John Berry

    Thread Starter Member

    May 17, 2016
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    Thank you very much for your quick reply.... :)
     
  4. John Berry

    Thread Starter Member

    May 17, 2016
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    Hello and thank you
    This is my actual battery, and this is the controller I was thinking of buying.
    What size solar panel would I need for this set up please?
    Thanks again. John.... :)
     
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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  6. hp1729

    Well-Known Member

    Nov 23, 2015
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    Size depends on the technology used. Figure about 0.1 to 0.25 Watts per square inch.
    How fast do you want the batteries to charge? Will the solar cells be supplying power to the load while the batteries are charging?
     
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  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    For battery selected I'd go for 20 V OC @ 15 W. Looks like a lot of controller for a small battery.
    In US the Harbor Freight 5 W X 3 panels with controller would be about right if on sale.
     
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  8. John Berry

    Thread Starter Member

    May 17, 2016
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    Thank you all very much for your replies, I really do appreciate it .... John :)
     
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Sorry John I led you astray. :(
    @Bernard has the correct size.:D
     
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  10. John Berry

    Thread Starter Member

    May 17, 2016
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    No problem, thank you very much for your help. John.... :)
     
  11. wayneh

    Expert

    Sep 9, 2010
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    Hang on a minute, this is way off track. Focusing on a tree and missing the forest.

    The size of solar panel you need doesn't depend on your batteries and charge controller, it depends on your load requirements versus the availability of incident solar light. If you live in a cave and want to power an air conditioner, no panel can help you. If you live in the Sahara and want to power a garden light, a single square inch panel might be enough. Your load, and the sun, doesn't care about your battery and charge controller. Those are part of an integrated solution but they are the tail of the dog.
     
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  12. Lyonspride

    Member

    Jan 6, 2014
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    That current rating refers to the maximum load current, not the charging current.
    The problem I found is that bigger capacity charge controllers tend to have a higher quiescent drain, I had a 10A that was taking 20mA from the battery and the solar panel (in UK weather) wasn't able to keep the battery charged due to the drain through the charge controller.
     
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  13. John Berry

    Thread Starter Member

    May 17, 2016
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    Ok, Thank you very much for your help... :)
     
  14. John Berry

    Thread Starter Member

    May 17, 2016
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    My load will be some sort of led lamp that I can have on during the night whilst asleep, at the moment I am having to leave the hall light on all night ( for the wife ) for about eight hours. I know that an led should run at 20mA, but It's what to put together. John... :)
     
  15. wayneh

    Expert

    Sep 9, 2010
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    Ah, OK, a single LED? As long as you have reasonable sunlight during the day, you should need only a very small panel.
     
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  16. John Berry

    Thread Starter Member

    May 17, 2016
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    I was thinking maybe 5 - 10 led's to get enough light. I have a usb lamp plugged in to my laptop now with nine led's that would be bright enough but I don't know its power consumption. Thanks John... :)
     
  17. John Berry

    Thread Starter Member

    May 17, 2016
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    And it runs on 5v not 12 obviously,,, :)
     
  18. John Berry

    Thread Starter Member

    May 17, 2016
    63
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    Thanks for the reply, I can see that there are a lot of factors to take into consideration... :)
     
  19. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    How about 3 parallel strings of 3 bright white LEDs in series & 150 ohm R. Good for 53 h ?
    Shrink panel down to 2.5 to 5 W & find suitable controller.
     
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  20. John Berry

    Thread Starter Member

    May 17, 2016
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    Hi Bernard, I think we might have got to the light at the end of the tunnel at last.
    Your idea sounds good, but only one 150r? where should it be placed? and I thought each series line should have its own resistor? Would the circuit look like this? Thanks John.... :)
     
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