# Solar cell charging circuit

I'm trying to build a very small LED light that runs of solar power. The goal is to have it charging most of the day and maybe get about 15 minutes of "led power" per day (comparable to these solar garden lights, with the exception that they don't need to run al night). I'm looking to power a single led at preferably high brightness (say 2V, 20mA).

Since I'm looking for a small form factor I ordered these, 1v 80mA, solar cells (25mm x 30mm). Preferably I would stick to one cell to keep it small. Now I'm not really sure if, and how, I would be able to store the energy collected by the solar cell and use it to power the LED at a specific moments. Some questions I have:

• Is it possible to use this a 1v solar cell to charge a battery? Maybe by boosting the voltage?
• What kind of energy storage would be good: Lipo, NiMH, capacitor?
• Are there circuit examples I can follow
• Can these circuits build in a cheap way?

Thanks

• so, what would be the LED you use? – Marcus Müller Apr 23 '17 at 19:26
• I'm not sure yet exactly, but say a standard, 2v, 20mA 5mm led. – Ruben Apr 23 '17 at 19:29
• Yes. Use a NiCd cell. No charge control is needed. Constant current is fine, and they tolerate over-charge as long as the current is low. You need some type of boost circuit to power the LED at night. – mkeith Apr 23 '17 at 20:37
• Take a look at this related question where the OP took apart a garden light and reverse engineered the schematic: electronics.stackexchange.com/questions/142242/… – mkeith Apr 23 '17 at 20:43
• Each one of those solar units you linked to seems to have two solar cells in series. There is no V-I curve, but I suspect the voltage will be high enough to charge a single NiCd cell. At least I think the idea is worth exploring for simplicity. I suggest you measure the open circuit voltage in bright sunlight. If it is something like 1.4V or 1.5V, it should work with a NiCd cell. – mkeith Apr 25 '17 at 4:47

I'm not sure yet exactly, but say a standard, 2v, 20mA 5mm led

So, 2 V · 20 mA = 40 mW power.

ok, let's round that up to 25 minutes:

25 min · 60 s/min · 40 mW = 60 J

Is it possible to use this a 1v solar cell to charge a battery? Maybe by boosting the voltage?

yes.

What kind of energy storage would be good: Lipo, NiMH, capacitor?

With the requirements above: probably simply a supercapacitor.

Let's say you charge that up to 5V. The energy in a 1 F supercap then is

W = 0.5 · C · U² = 0.5 F · 25 V² = 12.5 J

so, about twice as much as we strictly need, which is nice, since it gives us headroom for efficiency. You could of course use larger storage, which would give you more on-time :)

Also, supercaps have a high self-discharge rate, so this will only work if you want light shortly after the sun has gone down. If you want light hours later, better use a lithium or NiMH battery. These will have capacities that will let your LED run for hours, if not days. However, they might require more effort to charge. See the paragraph on dedicated ICs below.

Are there circuit examples I can follow

Certainly!

Can these circuits build in a cheap way?

Yes, and that answers the previous question, too: There's a lot of ICs designed to make charging of supercaps or batteries from small solar cells easy. I remember TI having a range of interesting chips, but I'm sure Maxim has some, as well as others, too. Their Datasheets and application notes will have good examples! Maybe, they even have simple eval boards. Or you can buy such pre-made on the internet.

Go into the power management / power harvesting category of these sites and look for solar charger circuits.

To drive the LED from the charged capacitor most efficiently, you'd probably use a switch-mode converter, too.

• Thanks for the explanation. Indeed a capacitor is not really an option since I want to store the energy for a longer period. About the boosting, do you have examples for doing this? And about the dedicated IC's. They seem pretty expensive ($5 at least), I want to make quite a number of these lights, so preferably look for a cheaper solution – Ruben Apr 24 '17 at 7:19 • @Ruben sorry, cost optimization is kind of something that I'd only care about in a commercial thing (if you plan to solder this stuff by hand, don't forget to factor in your additional build time for less integrated solutions with more parts). – Marcus Müller Apr 24 '17 at 7:25 • @Ruben 5$? Where are you looking? For all websites that I find carrying those, depending on order size, 2.50€ – 3€ << 5 \$ . For example, octopart.com/… – Marcus Müller Apr 24 '17 at 7:27
• @Ruben: And, if you're designing something new, then you should feel free to contact the manufacturer for IC samples. They like helping people build products out of their ICs! – Marcus Müller Apr 24 '17 at 7:43
• I was looking at farnell. But thanks for your input. I'll see where I can take it from here. – Ruben Apr 26 '17 at 9:03