Crunching the numbers: using solar cells indoors

[Mathieu] was interested in using more solar cells for his everday electronics. He looked around but couldn’t find much information about using pholovoltaic for small indoor devices. We remember hearing some of the same things from [Dave Jones] in one of his EEVblog installments from a few years ago which looked at solar calculators; the only indoor solar gadget we can think of.

The quest for knowledge was on and [Mathieu] decided to build this indoor solar cell test platform. It’s a stable piece of hardware that allows him to run reliable tests in many different conditions. It’s designed to compare two identical cells. One is charging the Lithium cell, the other is driving a load directly. A second battery powers the platform separately from the solar circuits so that it can be used as a data logger. He collects, dumps, and graphs info from his test runs, then discusses his findings. We won’t spoil it, but the results are not great. Mostly you need sunlight to get real results as it’s just so much more powerful than artificial lighting.


  1. xorpunk says:

    In a realistic scenario you’d have a charge controller and a data state storage method if data was involved, or termination handshake if signal was involved.

    I’ve been seeing people waste so much time over the years trying to figure solar’s a charging solution not a direct power solution..use it as such..

    • fartface says:

      it IS a direct power solution, you just cant be a panzy about it.

      I have a 90Watt single panel that runs a laptop directly just fine. It cost me $390.00

      Most people dinking with solar don’t have $20.00 in their pocket, Solar is EXPENSIVE. Once people understand that the rest is easy.

  2. Wouter says:

    Nice, that new chip is brilliant, although it would be a pain to get all those resistor values :(, nothing some SMD resistor stacking/rowing (parallel/serial) won’t solve though. I got stuck on this problem as well, there are no easy MPPT devices available, certainly none that will also keep an eye on the battery.

    The idea of just using a basic regulator which regulates to 0.75 times the open clamp voltage of a pair of solar cells pops to mind. When using a battery as a place to dump the current generated, won’t that work? And some basic shutdown feature of the voltage of the battery is too high (ultra low power comparator tied to shutdown pin?)
    The problem is that the regulator is all wrong, a normal switching regulator will reduce current throught the coil when the voltage is too high, in this case it should increase the current though. Anyone any idea how to solve that problem?

  3. Wouter says:

    Also, very nice of you Mathieu to invest time and money into researching this problem. Hopefully the patience pays of with some nice results.

    I would be curious to know how much better those solar gardenlight might perform using a chip such as this one.

  4. DanJ says:

    Interesting. I have applications for solar energy harvesting. I haven’t looked into it at all but have wondered if, for some low-energy devices, one could use a super or ultra cap in place of a Lithium Ion battery and simplify the circuitry (perhaps at the expense of even more efficiency).

    • limpkin says:

      DanJ: I tried it! However you’d need a lot of them to store the energy for a small device to run at least 2 or 3 days without sun. Leakage current is also important (in theory you can keep the charge for 1-2 weeks with super caps…)

      Wouter: you’re welcome :). Your idea is perfectly good. I guess you’ll have to adjust your step-up to keep your input voltage to the 0.75 value

      • DanJ says:

        Thanks Mathieu. Will look forward to seeing your data from this experiment.

      • nes says:

        I think 1-2 weeks is a bit pessimistic. I have built a device around a .47F super cap (a timekeeper) which lasted about 5 months between charges. Neither cap (Tokin FE series) or IC used are state of the art.

        It sounds like the BQ25504 can be set up to support super caps as well as Li-ion cells. The choice would come down to how much energy you need to store, what physical volume you have to store it and how much leakage can you tolerate.

        Excellent article. It’s good to have some practical figures for this stuff.

      • limpkin says:

        You may be right. I was mentionning what I had read on a 1.5F 5V supercap datasheet :)

  5. zigzagjoe says:

    Only successful application of indoor solar panels I’ve seen are the casio solar watches; I have the mtg950bdj myself and only put it in the sun on purpose once when I got it.

    Since then, for the last 6 years, it’s subsisted purely on what light it gets on my wrist (largely artificial)

  6. Mr. Hsu says:

    Don’t forget about the Google Radish project (featured previously, This is a great example of a practical application of indoor solar power that appears to work quite well.

  7. Neil says:

    Interesting interpretation of the phrase ‘we won’t spoil it but the results are not great’. Sorry guys, I kinda got the idea of the end result from that which, in my book, counts as A SPOILER.

  8. Chris says:

    Dont forget about this project for another use of indoor solar:

  9. mike says:

    dudes, have you heard of LTC3225? I’m designing a board with it now. I’ll post something when I have it.

  10. altzone says:

    My video on the solar cells mentioned above is here:


  11. John says:

    I’m working on a project to characterize indoor energy availability. We have extensive data for office environments and a few mobile measurements (including a walk through Times Square at night). The data is freely available here.

  12. Shady says:

    For better indoor collection organic PV (DSSC Dye Sensitized for one)is supposed to perform better with indoor light, given the correct chemistry inside.

  13. uhg says:

    I wonder if a similar approach could be useful for external solar power in a low light environment?

    I have toyed with cyclically charging some big caps directly from a solar panel, powering a boost converter + lipo charger once a sufficient charge has accumulated, then recharging the caps from the panel. Primitive and lossy perhaps, but it basically seems to work – though at this time of year precious little power is produced.

    I hadn’t thought of periodically sampling the open circuit voltage in order to optimize MPPT. Nice take-home from the datasheet!

  14. A. Karttunen says:

    You CAN use small solar panels (like those in photo, scavenged for example from garden lights),
    but to drive almost any load, you need to store
    the energy first into either a big electrolytic capacitor (>= 4400 microfarads) or a super-capacitor. In “BEAM-robotics” scene, these circuits are called “Solar Engines”.
    See for example:

    Some models need some special components, like for example:

    Also, my experience seems to indicate that although (mono-/poly-)silicon cells give more juice in direct sunlight, the thin cells are better (voltage-wise) when there is less light.
    That is, although they give very little current when not under direct sunlight, at least the voltage is enough to load the capacitor (slowly).

    Then, you can drive small motors and even solenoids, it just happens with less frequency,
    as most of the time the circuit is charging the capacitor.

    Have fun!

  15. tantris says:

    indoor solar is terribly inefficient.
    it isn’t about saving energy, but about convenience. ideal candidates to change to solar are gadgets that need a new battery only once or twice a year. (clocks, lcd thermometers, ..)

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