How best to find the "best" small solar panel of a particular size?


  • Hero Member

    Given that the advertised milliwatt rating of solar panels is hardly a reliable indicator (especially if the seller is on aliexpress or ebay or the like), what's a fair way to compare them? One could infer something about their relative power by measuring voltage and current under load (since dc power = voltage * current), but which load exactly? Or does it even matter? For instance, I have one small panel that, unloaded, appears to produce about 3.6v at a given light intensity, whereas the other (of equal surface area) produces 5v under the same conditions. As it turns out, the 3.6v one does a much better job of charging an ultracap with my solar harvester (https://forum.mysensors.org/topic/5274/powering-mote-24-7-using-only-a-supercap-and-solar/143), but I'm not sure if that's because it's 3.6v output instead of 5v (and each measures differently under load anyway). or because the panel is in some other way "better". The choice would be simple if I were only choosing between the two, but since I'm trying to optimize the panel chosen, I can choose from other units also on the market. Therefore, I'm trying to be smart, rather than random, about picking other panels to test before purchasing them.

    Any thoughts or suggestions?


  • Mod

    @NeverDie did you get anywhere with this? I'm building a low-power solar node and would like to know as well :)


  • Hero Member

    @mfalkvidd

    They're cheap enough that I decided to just try a few and compare empirically. A conservative yet simple test: if it can make a blue LED glow, then it's good enough to power your mote. It turns out that even very small solar panels can make a blue LED glow indoors with nothing but indirect sunlight, including this one: https://www.digikey.com/product-detail/en/ixys/KXOB22-01X8F/KXOB22-01X8F-ND/4840081
    Out of the panels I've tried, everything rated at 5.5v or thereabouts has worked, including on overcast days. That's important to me, because I don't want to assume direct sunlight.


  • Mod

    I have ordered a few boost buck converters that can output 3.3v with input from 0.9 to 6v. Let's see how they do with a couple of supercaps and a small solar panel.


  • Hero Member

    I've found that with a 5.5v panel of virtually any size, I haven't needed boosting in any room that has a window in it--even if the panel is pointed away from the window! To do a rough-and-ready site survey, try the blue LED test using a 5.5v panel and walk around your house with it . I think you'll be pleasantly surprised!


  • Hero Member

    Also, you don't technically even need a supercap. I find that I can power-up an atmega328p and send two packets from an RFM69W using a mere 100uF capacitor charged to 3 volts. With most solar panels, such a capacitor will charge up almost instantly. I think with some adjustments to my setup I can do the same using an even smaller capacitor if I wanted to.


  • Mod

    I want to make a small weather station so I want to be sure to have enough juice for the night as I increase the number of sensors. I found some cheap 2.7v 100F that cost as much as a 5V 4F, so I think I can have enough stored energy for a while.


  • Hero Member

    That should be plenty. I have a node that wakes up every 100ms and listens for 1ms each time to see if another node is trying to contact it. It also updates the gateway with its cap voltage level every 5 minutes. So, on average it's probably busier than your weather station. It can do all that for 24 hours on less than 5F charged to 2.7v. So, with 100F, you should have plenty of margin.


  • Mod

    Are you still using your solar Harvester to control the supercap charge? I am going to use a 5v panel so I'll have to use 2 supercaps in series and a diode.


  • Hero Member

    You mean the BQ25504? I dropped that quite a while ago. It was rather expensive in the first place, but it was also miserably hard to solder.

    One of the simplest circuits is just LDO plus diode plus one 2.7v supercap.


  • Mod

    What LDO did you use?


  • Hero Member

    This post is deleted!

  • Hero Member

    @gohan said in How best to find the "best" small solar panel of a particular size?:

    What LDO did you use?

    MAX8887EZK27

    However, if you're going to be ordering new parts, I suggest you wait a bit. I'll be posting a better circuit after I receive and test the pcb.


  • Mod

    I was just curious, since I already ordered the boost-buck converters. I only need to close the order for the supercaps and diodes


  • Hero Member

    The best deal I've seen is 10F supercaps on Digikey for $2 each, which would be good enough for most applications.

    The good news is that the pricing on supercaps is favorably non-linear. By that I mean you can, for example, purchase a good 400F supercap from Digikey for around $12, not $80.

    Which buck-boost converters did you buy?


  • Mod

    I was checking my orders and I forgot to order it... :confused:
    Anyway, this was what I was looking at: it was the only one I found with a low input voltage. That actually is the 5V version, but there was another one that had 3.3v output that I can't find it right now


  • Hero Member

    @gohan said in How best to find the "best" small solar panel of a particular size?:

    I was checking my orders and I forgot to order it... :confused:
    Anyway, this was what I was looking at: it was the only one I found with a low input voltage. That actually is the 5V version, but there was another one that had 3.3v output that I can't find it right now

    Well, the chart in the description foretells of a problem you're likely to encounter: to get 50ma of 5v output, you need to provide it with 260ma current at 1.2v. I don't know your setup, but I'm guessing your solar panel won't be delivering that. Will it scale to the available current, or just fail entirely? Who knows, because there is no datasheet. Of course, at 3.3v, it won't require as much current, but still.... you might want to look into that.


  • Mod

    of course, but I am counting for the supercap to supply the power and the solar panel to charge it during the day. With 2 100F supercaps voltage should never reach that low level within one night. I found the 3.3V version. The values in the table are just for reference, so you need to scale it down to the power required for an arduino pro mini and few sensors.
    I could also very well be that I may only need a voltage regulator to 3.3V because the voltage from the 2 caps would never drop below that.


  • Hero Member

    @gohan
    Because there are so many different solutions that work, it's a fun problem to compare notes on.


  • Hero Member

    @gohan said in How best to find the "best" small solar panel of a particular size?:

    of course, but I am counting for the supercap to supply the power and the solar panel to charge it during the day. With 2 100F supercaps voltage should never reach that low level within one night. I found the 3.3V version. The values in the table are just for reference, so you need to scale it down to the power required for an arduino pro mini and few sensors.
    I could also very well be that I may only need a voltage regulator to 3.3V because the voltage from the 2 caps would never drop below that.

    Well, given your approach, you may want to try this: https://www.openhardware.io/view/279/Adjustable-Boost-Converter#tabs-instructions

    Seems better than anything I see on AliExpress.

    Or this, which has a very low start-up voltage of just 250mv:
    https://www.openhardware.io/view/281/Solar-Energy-Harvester


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