9V solar USB charger project

Discussion in 'The Projects Forum' started by kje, May 6, 2012.

  1. kje

    Thread Starter New Member

    May 6, 2012
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    I`m working on a solar USB charger project and need some help on how to connect a 9V 2W solar panel, 4 AA battery, USB and digital voltmeter. The USB is for load (smartphone charging).

    Four 1,5v rechargable alkaline batteries in series is 6v, so I use a LM7806 regulator after the 9v panel, and then a diode to prevent the batteries from discharge into the solar panel. The voltmeter is for controlling the status of the batteries so I don`t overcharge them. How do I connect this together? Do I need to limit the current from the panel?

    [​IMG]
     
    Last edited: May 6, 2012
  2. wayneh

    Expert

    Sep 9, 2010
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    You've got some problems.
    1. The blocking diode will drop about 0.7V, so putting after the regulator will send 5.3V forward to the batteries. Too low.
    2. The regulator itself also will drop voltage, I think at least about 1.5V. Under much load at all, your panel voltage will drop and the regulator won't hold its regulation.
    3. You're going to have a similar problem providing regulated 5V for the USB output.
    4. You may or may not need charge regulation. My guess is you might get away with direct connection of the panel thru a blocking diode. But to test this you'll need details about your batteries and the charging strategy for the chemistry you're using, and about the current-vs-voltage curve for your panel.
     
  3. kje

    Thread Starter New Member

    May 6, 2012
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    Ok. Can I solve it to put the diode before the regulator?

    So it`s depending on how much load?

    What problem?

    The batteries are 6v, doesn`t I need charge regulation from the 9v panel?
     
  4. wayneh

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    Sep 9, 2010
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    1. That helps get better regulation, but doesn't solve the voltage drop problem.
    2. Very much. Google around for solar panel IV curves, to see what I mean. You'll see that for any panel, the voltage falls off gradually as current increases. When current is high enough to drop voltage by 20% or so, voltage falls off sharply with only a small increase in current. So even though your panel is 9V with no load, that voltage will drop quickly once charging is actually happening (current is flowing).
    3. Your batteries are capable of more voltage than the USB output should see, so the voltage needs to be regulated down. But you need 5V out, and may only have 5.5 in or less. That difference is the "dropout" and is too small to work. You might just use a diode to give a consistent 0.7V drop, which would probably keep you in safe USB range.
    4. Probably, but it depends on all those details. The panel in full sun will drive some current through the batteries, diode and regulator circuit. But the batteries MIGHT be able to tolerate that small (and hard to predict) current even when fully charged. Nicads are the best for tolerating such a trickle. The better approach is to use a genuine charging strategy.

    You might like to read about the Minty Boost project. You'll learn a lot about some of the issues, although it doesn't use a solar panel.
     
  5. kje

    Thread Starter New Member

    May 6, 2012
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    So it`s going to be a voltage drop at a particular load threshold? How do I find out this threshold? All those details you are talking about is mentioned here http://ladyada.net/products/usbdcsolarlipo/design.html am I right?

    I`m not so into electronics, so I don`t understand exactly everything... Is there a good video tutorial about building a project like this?
     
  6. kje

    Thread Starter New Member

    May 6, 2012
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    Here is some pictures of my solar charger project. I`m trying to connect a 9V 2W solar panel, schottky diode 5819, LM7806 regulator, on/off rocket led switch, digital voltmeter, 4 AA 1,5v rechargable alkaline batteries and a USB.

    The USB will be for load (smartphone charging), but I haven`t connected the USB yet so I don`t know if the solar panel are charging the batteries now... From the solar panel I soldered in a diode to prevent the batteries from discharge into the solar panel. After the diode I use a LM7806 regulator to step down the voltage to 6v so it can charge the four 1,5v batteries which is 6v in series. On the 7806 regulator I added a boiler plate flange for cooling. From the regulator it goes to the on/off switch which has three points; 1) power, 2) load and 3) ground. The voltmeter is for controlling the status of the batteries so I don`t overcharge them. Is the wiring correct so far?
     
    Last edited: May 7, 2012
  7. wayneh

    Expert

    Sep 9, 2010
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    You mean it's monitoring voltage, so you can manually avoid an overcharge, right?

    Anyway, yes, it looks OK. You should be able to detect charging by putting the panel under bright light versus shaded. The battery voltage should tick up a bit when it's charging, down when draining.

    You should consider a cheap (<$5) meter that you can use without wiring it in. Your little meter might be using all the power the panel makes and more. This would allow you more measurements and a better understanding of operation. [edit] Oh wait, it looks like I see one in your picture.

    To "regulate" for USB, consider just another diode from the battery to the USB port. At your current (unloaded) voltage, it would be almost perfect. You may also want to investigate low dropout regulators.
     
    Last edited: May 7, 2012
  8. kje

    Thread Starter New Member

    May 6, 2012
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    Yes! :)

    I tried to measure current between the voltage regulator and the on/off switch (with the switch on). I measured 0,4ma with the panel under bright light from a lamp. I don`t get this... I did this at the night when it`s dark outside, but the lamp is really bright! Shouldn`t I get more than 0,4ma? Am I doing something wrong with the wiring? (Look at the picture) With the lamp off (with just normal room light) I measured -3,5ma, this is maybe reverse from the battery?

    I`ve wired in a cheap digital voltmeter as you see in the picture. I hope it don`t draw very much power...

    Yes, I will add another diode between the battery and the USB. When I am finished with this project I want to investigate low dropout regulators.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Look at its datasheet. I bet it's more than you think, at least 10mA.

    You shouldn't be able to get -3.5mA with the blocking diode in place. Can you show a schematic of what/where you were measuring? Oh, maybe you were seeing reverse current into the regulator? Not good.

    You should measure panel voltage (after the diode) and battery pack voltage under your "bright light". This will help sort out what is happening, whether the regulator has enough room to operate. My guess is that it does not, except maybe in full sun.
     
  10. kje

    Thread Starter New Member

    May 6, 2012
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    I tried to make a schematic of the project (without the voltmeter and USB connected). Can you check if I wired correct?

    I measured current between the on/off switch and the regulator. (cutted the red wire and had my multimeter between)
     
    Last edited: May 7, 2012
  11. wayneh

    Expert

    Sep 9, 2010
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    I'm pretty sure your switch will be shorting your panel - through the regulator - when in the off position. You should make your ground connections somewhere else, not on the switch terminal.
     
  12. kje

    Thread Starter New Member

    May 6, 2012
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    Where can I make my ground connection?
     
  13. kje

    Thread Starter New Member

    May 6, 2012
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    I found out a voltage regulator is kind of wasteful since it will convert the difference between the voltage in and voltage out to heat. So I will use 3 cells (4,5v), charged by the 9v panel, connected to a step up converter to provide the 5v for my USB devices. I might consider paralling two sets of 3 cells. Still 4,5, but, twice the capacity. I`ll get more energy from the cells using the step up converter than using a voltage regulator. I have to order the parts.

    In the meantime I tried to build a simple solar charger with just the diode and a 7805 regulator I had. This is without batteries (see picture below). Everything is glued on the panels backside. I measured 4,88v in bright light. But it didn`t charge my USB speaker, or power up my USB keyboard... (wich powers up when connected to my laptop USB). Any thoughts about why this didn`t work?
     
  14. wayneh

    Expert

    Sep 9, 2010
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    Sound like your strategy is improving. Your changes make good sense.

    Your setup "should" have worked for many devices. Two things to check. First, see what happens to the voltage when either of your devices are actually connected. Starting at "only" 4.88, it may have dropped out of range as soon as a load was connected. Second, check if those devices require anything on the data pins, whether they will power up with only the power pins active. Some things won't although I would expect your speaker to work.

    Oh, one more thing. Check and double and triple check you have the polarity correct at your USB adaptor.
     
  15. kje

    Thread Starter New Member

    May 6, 2012
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    Thank you! :)

    I did as you said and measured voltage when my USB speaker was connected. I measured 4,33v. When I connect the speaker to my laptop a red light indicate charging, but when I connect it to the solar panel its a blue light (like when it`s on). I guess that means it give power to the speaker but not enough voltage for charging?

    I guess my USB keyboard require something on the data pins.

    I also made a second simple version of the solar charger, but with the 7806 regulator (the left one in the picture). This version didn`t manage to charge the USB speaker either (you can see the speaker to the left in the picture as well). I measured 6v from the USB without any load, but with the speaker I measured 4,38v (almost the same as the first solar version). My hope was this version would have higher voltage as I used the 7806 regulator...
     
    Last edited: May 9, 2012
  16. wayneh

    Expert

    Sep 9, 2010
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    Have you looked at the output of your panels into a load of, say, 10-100Ω in full sun versus under your light? Your light may only be a fraction of full sun. Unloaded voltage isn't useful - you need to test under a load of 50mA or more. If your voltage drops below 4.5 in full sun, you'll need more panel area.
     
  17. wayneh

    Expert

    Sep 9, 2010
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    You might be able to see if others have discovered this. The tricks are well know for iPods and iPhones, but a keyboard may have far fewer experimenters working on it. That said, you could try using resistor dividers to put various combinations onto the data pins, eg. 1 and 4, 2 and 3, 3 and 2, 4 and 1. That's just a few quick experiments and if they fail, the approach is probably a dead end.
     
  18. kje

    Thread Starter New Member

    May 6, 2012
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    Both the simple versions of the solar charger are successfully charging my Nokia phone and my friends Samsung Galaxy in the sun. :)

    Is it ok to connect a 9V solar panel to three 1.5v alkaline batteries and a DC-DC 3 to 5v step up converter? Will the converter be broken with the 9v solar panel?
     
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