How best to find the "best" small solar panel of a particular size?
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Also, Pololu sells a pre-made one that can take an input voltage of 0.5v and has an adjustable output voltage: https://www.pololu.com/product/2560
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The one from aliexpress is only step up, the other it's interesting but I don't need the variable output voltage since pretty much everything runs at 3.3v.
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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. -
@NeverDie Wow, that digikey solar cell is tiny at 7mm x 22mm! I'd love to see one of these worked into the plant monitor, as really that only needs to transmit a few times during the day at most and would be fantastic without need of batteries.
@tomtastic said in How best to find the "best" small solar panel of a particular size?:
@NeverDie Wow, that digikey solar cell is tiny at 7mm x 22mm! I'd love to see one of these worked into the plant monitor, as really that only needs to transmit a few times during the day at most and would be fantastic without need of batteries.
Yes. And since plants need light....
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@gohan I think you may run into a problem with your step-up design. From a cold-start with dim light, very likely the step-up will drain current from your buffer capacitor faster than it's being added, and it will just spin its wheels and produce no usable output. Please do let me know if that's NOT what happens or if you have a design which avoids that happening.
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I thought about that too, that's why I choose to use big capacitors, so that voltage would never go that low.
In that case I think there should be some components that are able to wait a minimum 1volt or such before allowing current to pass through -
@NeverDie did you get anywhere with this? I'm building a low-power solar node and would like to know as well :)
@mfalkvidd said in How best to find the "best" small solar panel of a particular size?:
@NeverDie did you get anywhere with this? I'm building a low-power solar node and would like to know as well :)
The truth is worse than I imagined. For instance, I purchased four of these panels:
https://www.aliexpress.com/item/6V-0-6W-Solar-Power-Panel-Poly-DIY-Small-Cell-Charger-For-Light-Battery-Phone-Toy/32573510541.html?spm=2114.13010608.0.0.QVU7RI
and measured the open circuit voltage of each under the same deliberately dim ambient room light. Each had a different open circuit voltage: 3.2v, 2.7v, 2.5v, and 2.2v. That's quite a spread in performance! I'm not sure what accounts for the difference, but you can't really guess just by looking at them which would be better or worse. So, really, there's no way to predict from the advertising alone what you're going to get. -
@gohan said in How best to find the "best" small solar panel of a particular size?:
what did you expect from the quality checks in China? 😁
Well, to be honest, what I expected was that they would all be roughly the same, but probably far short of what was claimed.
Where are your solar cells made? China, right? How are you dealing with the variation in what you get? I could buy solar cells from Digikey that are much more uniform, but they'd probably cost 6-10x as much. So, should I just buy more than I need from China and bin them myself? Or should I change the design to assume the worst of the possible range? Or buy from a seller with a better reputation? I think I'll try the later first.
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Interestingly, in direct sunlight they all measure at an open circuit voltage of 7v +- 0.1v. So, go figure as to why the range is so much wider under dim conditions.
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Interestingly, in direct sunlight they all measure at an open circuit voltage of 7v +- 0.1v. So, go figure as to why the range is so much wider under dim conditions.
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@mfalkvidd I doubt it. I measured it more than once, and it was repeatable.
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@mfalkvidd I doubt it. I measured it more than once, and it was repeatable.
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Anyhow, I'm going to string a number of these cheap panels in series and see if I can power a mote off of mere moonlight. I think it will probably work, which would be pretty cool.
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@mfalkvidd
Will do. Have you found a good source for mini/micro solar panels? Not being confident about what I would actually get, to date my purchases have been somewhat random.I think the Solar Bit would probably be excellent for moonlight powered motes, if only because their combined size would still be small enough so as not to be awkward. Of course, being powered by moonbeams is an artificial objective. What it would really mean is that your mote could probably be powered just about anywhere that's not completely sealed off from light. Any indoor living space that's not a closet would probably get enough light.
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@mfalkvidd
Will do. Have you found a good source for mini/micro solar panels? Not being confident about what I would actually get, to date my purchases have been somewhat random.I think the Solar Bit would probably be excellent for moonlight powered motes, if only because their combined size would still be small enough so as not to be awkward. Of course, being powered by moonbeams is an artificial objective. What it would really mean is that your mote could probably be powered just about anywhere that's not completely sealed off from light. Any indoor living space that's not a closet would probably get enough light.
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@NeverDie I bought my first two over a year ago. Have not used them yet. So no, nothing useful unfortunately.
@mfalkvidd
It's a pity this topic hasn't attracted more interest. I guess people are happy with their batteries. -
There seems to be a sweet spot at 6V 1W when it comes to price/performance, at least specified performance. Smaller panels are relatively expensive, and so are larger. I have ordered four of these but they haven't arrived yet.
I have been looking for flexible panels, something that could be folded or rolled up, but haven't found anything with a decent price so far.
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I think that's because that's where the market demand is. It's hard to find smaller panels generally. I have these on order, but neither has yet arrived:
https://www.aliexpress.com/snapshot/0.html?orderId=83597824322273&productId=32253274314
https://www.aliexpress.com/snapshot/0.html?orderId=83519564882273&productId=32644217312
