I have eight solar lawn lights in my yard, and sitting under the sun for several years has taken its toll. Even though they're basically disposable, I thought I would play with them before throwing any away. I tried to retrofit one with a super capacitor and the attempt taught me several problems not the least of which is that those supercapacitors I bought were too big fit inside. I had better results from my first experiment swapping out a NiMH battery. For the foreseeable future, I think that's the way to go.

Since my capacitor test light turned out to have a dead solar cell, I used my bench power supply set to 2V as the power source. I charged it up during the day, then disconnected power at sunset to see how long it ran. It shut off a little over six hours later which is roughly the runtime I want out of these lights. It's also on par with what I get out of running these lights on salvaged NiMH batteries past their prime.

I had contemplated trying my supercapacitor test again with smaller capacitors that would easily fit inside, but physically smaller capacitors would have less energy storage capacity as well. Which means they can't run as long, and my light would go out sooner. I could compensate for this by wiring several smaller units in parallel, distributed around the light's interior instead of one big cylinder, but then cost would go up.

The capacitors I bought were advertised as 500F. Given the realities of no-name Amazon vendors I doubt that number is accurate, but it is a starting point for comparison. There are smaller capacitors available roughly the size of my salvaged NiMH cells, which I know would fit with minimal trouble. Maybe even a pair of them. The highest-capacity units I found at that size (*) were advertised as 100F and cost more than my "500F" units. If it runs the lights for 1/5 as long, the lights would only illuminate for a little over an hour before going out. Even a pair working in parallel would go dark in less than two hours, and that's too short.

I would expect supercapacitors to withstand daily charge/discharge for many years with minimal degradation. But as things stand I would have to pay a price premium and give up significant runtime and even then the solar cell may die first. I don't think that tradeoff makes sense so I've decided to stay with NiMH batteries for now and possibly reevaluate supercapacitor price/performance again in a few years. Especially since I discovered past me had stashed a batch of lights I can use today.

(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.