What solar panel and battery to select?

Discussion in 'General Electronics Chat' started by spinnaker, Oct 9, 2012.

  1. spinnaker

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    Oct 29, 2009
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    I'm dragging my solar powered entry light out of the closet. Just as a background I designed a Pic based controller to monitor sunlight to determine when to turn the light on and off again. It also monitors voltage levels of the battery and turns the light off when the voltage level is low.

    It has an off the shelf charge controller and I am using an off the shelf buck puck to power the light.

    The light itself takes less than 700ma. I have not calculated the consumption of the pic controller and charge controller but I can't imagine it amounts to much.

    The panel is an old panel that has been around for years and needs replacing.

    The whole thing is working great with the exception that that duration that the light is on is slowly decreasing. I suspect that the battery is becoming sulphanated. The battery is much more than a year old. Also I have the voltage level to turn off the light at 10V and I am thinking that is also taxing the battery.

    The current battery is a 12 6AH SLA.


    I'd like to replace the panel and battery.

    I was thinking this
    http://www.amazon.com/Instapark®-Bl...8&qid=1349794737&sr=8-21&keywords=solar+panel

    for a panel and maybe a 18 to 24AH SLA

    Is this a good choice? Is it overkill or should I go with a higher wattage panel?

    My thinking is that we have days where there is little sun and it would be nice to be able to maintain a charge enough to light the light over a couple of days with little sun without taxing the battery much.
     
  2. Maynardr6

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    Oct 9, 2012
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    I helped my friend design a pack with these http://dx.com/p/trustfire-protected-18650-lithium-battery-2400mah-2-pack-gray-5776?item=51. He needed something lightweight to charge his ipad while camping. It had a solar panel too, but no charge controllers or anything. I figured with the size panel he had, it would never exceed the charge rate as he had it configured. The cells have circuitry built in that shuts them off when charged and when discharged. You won't be carrying it around, but there are other advantages. With SLA, you really want to use only half of its capacity or the life will suffer. The lithium ones, you can use the whole rated capacity whenever you like. You can get 7.2AH for $20. I'm kind of a flash light and rc junky, so I'm kind of partial to lithium. Just throwin that out there.
     
  3. spinnaker

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    Where do you find a battery pack that you can easily connect wires? Most I have seen are those plug in replacement packs, unless I can get the receptacle somewhere installation can be difficult. Or if I can get standard D cells then I guess that would do.
     
  4. spinnaker

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  5. Maynardr6

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    Oct 9, 2012
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    Yea, I like that one. It doesn't say "protected" but that $0.99 charger implies that it is. I also found this holder http://dx.com/p/14-8v-4-x-18650-battery-holder-case-box-with-leads-103855?item=4. I can't remember what we finally decided on for my friend, but I also found this which is waterproof. http://dx.com/p/4400mah-external-battery-pack-with-pouch-for-sku-29489-30864-4-18650-32756?item=4. The only thing that would sway me more towards the individual cells is that I'm familiar with them, and the in the world of 18650 cells, the word "protected" can be somewhat ambiguous. Modes of protection are: over current, under voltage, over voltage, over temperature. "Protected" isn't an all inclusive word, so I like to order things based on the reviews, especially on dealsextreme. I think in my friend's pack, we used 12 cells in a 4 x 3pack configuration for 12ish volts. They charge to 4.2v and discharge to 3.2ish volts and they're 3.7v nominal.
     
  6. Maynardr6

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  7. spinnaker

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    What $0.99 charger? Or do you mean the walwart in the picture?
     
  8. spinnaker

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  9. spinnaker

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  10. ErnieM

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    I'm overlooking a solar installation we use on a lighthouse. One curious thing it uses is a direct connection from the solar panel to the sealed lead acid battery bank. The panel is 150W max and the batteries have a 300AH rating. (I'm presently looking into adding a real MPPT charging system during the next upgrade.)

    The trick is to balance the power in with the power out while leaving a substantial charge in the batteries. SLA, even deep discharge types, last longer the less current you draw before a recharge. An extreme example is the USCG: they will spec a battery bank with 30 times the daily load draw. A 50% draw is probably all you would use, but 25% is better.

    Depending on where you live you will have some long winter nights with short days, so the light may be on some 14 hours, but only get a good charge for 5 hours. Charging isn't perfect, so let's assume your charge is 70% efficient. Therfore:

    Load:

    .7 amps times 14 hours is 9.8 Amp Hours.

    Battery:

    load is 50% to 25% of the battery capacity for the battery should be:

    Min: 1/.5 * 9.8AH = 19.6 AH

    Better: 1/.25 = 39.6AH

    Panel:

    At 70% effieciency, we need 1/.75 * 9.8AH = 13AH

    And we need to accumulate this in 5 hours:

    13 AH / 5 Hours = 2.6 Amps

    So the panel needs to deliver 2.6A at 12V or 31.2 Watts.
     
  11. Maynardr6

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    Now I'm seeing a comment about the charge indicators on what I thought was just a walwart. I might "smart" to some degree or maybe it just indicates that current is flowing. More research on that one is needed. You can parallel the 18650s. The neat part about having batteries that are individually protected like the trustfire ones is if you put 3 in paralled and they are unbalanced, they will individually stop charging and more or less self balance. The comments on that pack mention short protection. We still don't know for certain if they're over voltage protected. You can also hook them directly to the panel and everything will just take care of itself at least in theory. I will do some experiments and get back to you.
     
  12. Maynardr6

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    Ok, my experiment suggests the trustfire protected cells will work like I thought even connected to a 19v panel. I set my power supply for 20v and connected it directly to the cell (this is fine if your panel can't put out more than the battery can handle). This is a smaller cell than the 18650s, so the 500ma current limit in the power supply was appropriate, but you don't have to worry about that. When the cell hits its charge voltage of 4.2v, it shuts down. I turned voltage down to 0v and back up to 20v (cloud simulator :). It charges for a couple seconds and shuts down again, all perfectly fine. This is why I was recommending the trustfires. If the blue pack thing has overvoltage protection, it will work in the same way and you can hook as many as you like together in parallel. Clip of the test:
    http://youtu.be/2aVm-wNJX-k
     
  13. spinnaker

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    Thanks Ernie. Considering the fact that I need at least 20AH, should I stick with SLAs? Also have a charge controller already that I know that works with SLAs.
     
  14. ErnieM

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    If you have a good controller then why not use it?

    On the other hand, the batteries may be more expensive part of the system. I did see some small SLA batteries at Digi-key once when looking at a replacement for my lawn trimmer: I forget the specs but it was about 30-40 bucks for one battery; more then the trimmer was worth so I just bought a new one.
     
  15. Kermit2

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  16. spinnaker

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    Thanks



    I just picked up a 22AH from New Egg for $44.

    Maybe order another later on down the road and a desulphator. But I suspect with the larger battery and larger panel I won't be going through batteries as much.

    As I mentioned, I have the ability to set the voltage level at which the light is turned off. Any suggestion on what level I should use?
    I can set the level in .5 volt increments from 6 to 14V. Of course I have the code for the Pic so I can change those increments if needed.

    I might look into putting the Pic to sleep too but I wonder if I really need it.
     
  17. Kermit2

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    12.0 volts would give you the longest number of 'cycle' lifetime.

    11.5 volts will give less and the lower you set it the bigger the difference becomes.
    Let it go dead, or get lower than 10 volts very often and it will die soon and needlessly.

    12.0 volts represents a considerable amount of discharge if the drain has been one of many hours and light current.
     
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