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Solar Power Supply

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    • #1321
      Kevin Young


        I am looking to make my own solar charging circuit for a remote datalogging system. I have a 3-AA battery holder and I am planning on using 3 AA NiMh 1.2V batteries to be placed in the battery holder. I am also planning to use a 1N5817 blocking diode, so current does not get drawn back to the solar panel.

        If the batteries are wired in parallel, then the total voltage for the battery pack will be 1.2V, 2300mAh * 3 = 6900 mAh. I could also wire two of the batteries in parallel and 1 in series for 2.4V, 4600mAh.

        I’ve seen this configuration table around the internet listing the recommended solar panel output voltages to go with the voltage of the battery pack:

        Configuration Relationships between solar panel and storage battery
        Storage Battery -> Solar Panel
        1.2V ->    2V ~ 2.5V
        2.4V ->    3.5V ~ 4V
        3.6V ->    5V ~ 6V
        6V ->    7.5V ~ 9V
        12V ->    15V ~ 18V

        According to that, it says that for a storage battery of 1.2V, you need a solar panel that outputs 2-2.5 V, and for a storage battery of 2.4V, you need a solar panel outputting 3.5-4V.

        My questions are:
        1) Is it correct that I would need a 2.5V panel for a 1.2V battery pack? I did some research online and I can only find 2.5V panels from China (for e.g. here: http://www.aliexpress.com/item/High-Quality-0-5W-2-5V-Solar-Panel-Solar-Cell-DIY-Toy-Panel-Polycrystalline-Solar-Cell/32435616123.html), I couldn’t find them anywhere from a US supplier.
        2) Would I need a solar charge controller? If so, why? According to this link: http://www.solar-electric.com/solar-charge-controller-basics.html/, if the panel outputs 2w or less, you wouldn’t need a charge controller.
        3) If I use a 2.5V, 200mA peak current panel (like the one I linked above) with a 1.2V, 6900mAh battery pack, I would plan on using 3 of these panels in parallel for a peak current output of 600mA. Does this seem reasonable? Or should I use more panels? 600mA would be a little less than 10% of the battery capacity, which I have read you shouldn’t exceed.

        With 3 of these panels and a 1.2V, 6900mAh battery pack, I would expect that it would take 13.8 hours to charge the battery, probably under ideal conditions. My calculations are below:
        (8.28 Watt-hours / 1.5 watts) * 2.5 (loss factor) = 13.8 hours

        That formula was taken from this page: http://www.voltaicsystems.com/blog/estimating-battery-charge-time-from-solar/

        Any other feedback and suggestions are very much appreciated. I just don’t want to start ordering parts and find out that I planned erroneously :). Thanks!

      • #1324

          Hi Kevin-

          You’ll probably want to keep the batteries either all in series or all in parallel to keep them charged correctly. I haven’t done a lot of work with batteries, but what I’ve seen seems to have them either all in parallel or all in series. There’s a pretty similar circuit here


          with some nice discussion below. Based on reading that, it looks like your system will work OK, but it’s pretty inefficient, and not something that will scale up really well. There’s some interesting reference information here


        • #1340
          Danny Waz

            Before I get to the questions, let’s look at the battery issue. Reading between the lines, I’m assuming you actually want a 1.2 V power supply, not a 2.4 V supply. If that’s the case, then yes, wiring your 3 AAs in parallel will work and increase the capacity. That said, I’m curious as to what you are powering off of 1.2 V. Any Arduino-based system would require at least 3.3 V and likely 5 V.

            As for the parallel/series combination idea, I would not use the configuration you suggested to get 2.4 V. It would produce 2.4 V when first plugged in, but the batteries might charge/discharge at different rates. Series/parallel combinations almost always comprise one series in parallel with an identical series. Laptop batteries, for instance, often combine cells in series to get the proper voltage, then put two or more identical series in parallel to up the capacity. In your case, doing that would require four batteries (a 2-cell series in parallel with another 2-cell series), and please make sure you use identical batteries and change them all at the same time.

            An alternative is to make your power infrastructure (batteries, solar panel) at a higher voltage (say, 3.3V or 5V) and use regulators for powering your devices. This, of course, can be inefficient (less so if you can find appropriate switching regulators), but you might make up for that with the wider availability of off-the-shelf products at standard voltages.

            Now, your questions:
            (1) Yes, but it’s not an exact science. You want your panel to be rated slightly higher than your battery voltage to account for overhead and so that you can still charge in less than perfect, cool, sunny conditions. A 1.8 V panel would still charge your 1.2 V battery pack under sunny, cool conditions. a 3 V panel would charge under less ideal conditions but might necessitate some circuit protection if you are powering delicate electronics.
            (2) Probably not. A charge controller essentially regulates the panel power into the battery and prevents the battery from being over-charged. Charge controllers are usually used in bigger setups involving high-capacity, 12 V marine (deep-cycle) batteries. For low-voltage/low current applications without delicate electronics (think solar path lighting), it’s not necessary at all. You might want to look up basic voltage protection for your data logger, but the batteries will probably be okay without a charge controller circuit. (Note: I’m sure there are experts on this topic on this forum, and I would welcome any additional comment from experience)
            (3) That seems reasonable as a starting point. Remember, you do not need to charge from zero to full capacity every time the sun is out. The goal is just to have more power going in than going out on average. If you find that it dies in full sun, add solar panels in parallel. If you find that it works in full sun but dies after a few cloudy days, add batteries in parallel.

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