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

[gohan]

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.

[NeverDie]

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

[NeverDie]

@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

[NeverDie]

Also, do you have any particular reason you want to boost to 3.3v? If not, then if using an RFM69W, it would be more efficient to boost to a lower voltage, like maybe 2v.

[gohan]

Not really, it's just the standard voltage for Arduino, radio modules and sensors. I did look at that project, but smd is still unknown to me. 😌

[NeverDie]

Here's a link to a basic LDO 2.7v supercap solar charger:
https://www.openhardware.io/view/382/Basic-27v-Supercap-Solar-Charger

[NeverDie]

@gohan
I found a module from the same seller, but it can convert from 0.7v:
https://www.aliexpress.com/store/product/0-7-5V-to-3V-3-3V-5V-DC-DC-Boost-Converter-voltage-Step-up-Module/2348129_32800430445.html?spm=2114.12010615.0.0.SYUdHL

[NeverDie]

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

[gohan]

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.

[NeverDie]

@gohan
Good luck with it then. Let us know how it works out for you.

[tomtastic]

@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.

[NeverDie]

@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....

[NeverDie]

@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.

[gohan]

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]

@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]

what did you expect from the quality checks in China? 😁

[NeverDie]

@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.

[NeverDie]

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.

[mfalkvidd]

@NeverDie couldn't the variations be due to small changes in lighting when you tested?

[NeverDie]

@mfalkvidd I doubt it. I measured it more than once, and it was repeatable.