Solar powered Lithium-Polymer Battery supplemental charging circuit...different twist

Discussion in 'The Projects Forum' started by nanobot9000, Nov 26, 2008.

  1. nanobot9000

    Thread Starter New Member

    Nov 26, 2008
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    First of all I am just a hobbyist in electronics and am self-taught - thus my problem! I like to work a lot with robotics. I just joined the site and did some searching, but thought I should post my request for assistance.

    I need help designing a simple circuit to charge a 3.7V/20mA LiPo cell from a 4.2V/18ma solar cell. I have researched various charging chips and voltage doublers, but either I find overkill or the two together do not work well. I have an AC powered charger that would work to completely charge the cell, but this design request will be used in a small robotics platform and I thought that while it was just sitting there - any ambient light could give it a little bit of a charge. I know this is not optimal, but again this would be a supplemental charger. Since the solar cell at times only puts out 1.8 to 2.2 v at a distance of 2 feet from a light, I would think I would need some sort of charge pump/super capacitor to help out. The other major design consideration is that it needs to be as small a possible and fit within a 1" X .5" space. I planned to use SMT parts for the final design, but could use other parts for protoyping.

    I realize this is a lot to ask, but I am hopeful that someone will be willing to help me out.

    Thanks,

    Mike
     
  2. nomurphy

    AAC Fanatic!

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  3. scubasteve_911

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    Dec 27, 2007
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  4. nanobot9000

    Thread Starter New Member

    Nov 26, 2008
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  5. nanobot9000

    Thread Starter New Member

    Nov 26, 2008
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    Thanks - I have put together a few circuits using various samples of charger and voltage pumps from Linear, but can't seem to get the right combination. Most charge chips want at least 3V. I was looking at chargers that did not have inductors, but might have to resort to using them. Any and all thoughts are appreciated. Thanks
     
  6. scubasteve_911

    Senior Member

    Dec 27, 2007
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    You're welcome. You will need an energy storage element of some sort, whether it be capacitive or inductive. This is because you need to store voltage and add it to the incoming voltage when your cell voltage is smaller than the charging voltage.

    Steve
     
  7. nanobot9000

    Thread Starter New Member

    Nov 26, 2008
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    Ok that is helpful. Here is a circuit designed by Wilf Rigter for BEAM robotics group that I have used successfully with a 6v solar cell. Any tspecific houghts on tweaking the circuit to function with lower output solar cell like I mentioned above. 1381 voltage detectors are a bit hard to come by so any other part substitutions would be appreciated. As I am only really good at turning schematics in to circuit boards, but not actually designing them, I would greatly appreciate any specific part recommendations and placement.

    Thanks,

    Mike
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    When a voltage doubler doubles the voltage then the output current is half the source current.
    If the voltage of your solar panel is half its rated value then its current must be extremely low and after voltage doubling will be almost no current to charge your battery.

    The solar panel is spec'd with sunlight. Is your lightbulb the same brightness as sunlight?
     
  9. nanobot9000

    Thread Starter New Member

    Nov 26, 2008
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    Thanks and a very good point. The light bulb was a 100watt Grow bulb and I have also tested with a halogen bulb at a distance of 1 foot and get anywhere from 2.2-2.4V at 6inches it goes to the full output. I know I would have better performance from natural sunlight. What about some combination current amplifier/voltage doubler - or is that the holy grail of portable power wishes? I know that is probably asking alot, but that tends to be what I do. Do you have any thoughts on the attached circuit as a starting point for a new design? I have been able to charge a cell after about 12 hours in the sun with a 6vt cell- while not optimal - it is better than just sitting there, but my current design needs a small solar cell footprint.

    Thanks,

    Mike
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    It looks like the amount of light is too low for your small solar panel to do anything.
    An amplifier needs to operate from a source of power but you don't have any power.
     
  11. nanobot9000

    Thread Starter New Member

    Nov 26, 2008
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    I am attaching the test results and specs for various solar cells sold by solarbotics. The specific cell I am using is the 2433-B/MSE from their website: "Our SCC2433-B solar cell is a monocrystalline cell offering 4.5V (open circuit) and 18mA (short circuit) in a 24x33mm package." Any thoughts on tweaking the circuit I uploaded previously to work with a smaller cell?
     
  12. Audioguru

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    I noticed that the results of the tests were with the solar cells in direct sunlight at noon.
    You have a dim little light. Your solar cells won't do anything and it will be worse if they are smaller.
     
  13. nanobot9000

    Thread Starter New Member

    Nov 26, 2008
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    OK - and that was just a quick test - by placing it closer to a light I can achieve the outputs in the graphs. Given that I could achieve the full 4.5 volts - how could I design a circuit or adapt the one I attached to use maybe some form of small storage cap to charge the liPo. Maybe something where once the LiPo voltage drops to say 3.00 volts charging begins to raise the cell back to 3.7volts. I have found an ultra-low voltage operation charge pump IC from Seiko (S-882Z) that can be used with a step-up DC-DC converter startup (S-8353). I have also been looking at EnerChip from Cymbet - although not sure that is commercally available. Even if I can harvest light an store it for a period of time. I know LiPos are more sensitive to charging, but I am hopeful. Thoughts anyone?
     
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