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Comment by ethagknight | Hacker News Reader

I dont understand why the little solar panels that were included on every calculator from my childhood aren't used more broadly. I would imagine a little solar strip combined with a coin cell could extend the life of the coin cell considerably in applications like an AirTag, especially if premium functionality is only offered when sola power is available (like software updates or LED status displays, etc. seems like there should be a world of potential combining lithium cells, combined "fire and ice" cores, and little solar flaps.

I guess obviously not or we would see a lot more of it?

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Your comment got me curious, so I did some googling. The answer is pretty straightforward: those calculators use ridiculously little power. They have bytes of memory, use the simplest LCD displays, and are idle essentially 100% of the time. Moreover, the assumption is that you're not working in the dark. The solar strip is essentially exactly the size needed to run the calculator: as soon as the light goes away (or even gets dim) the device powers off. For a light switch, that's kind of exactly what you don't want. A cross country flight in the cargo hold of an airplane would be enough to pretty much drain a solar air tag: a thief would just need to put your luggage in a black garbage bag to render the air tag useless.

I suspect you'd need a panel that's a few inches large for it to charge faster than a device like an air tag could draw power. But again, that assumes it'll routinely see some fairly bright light.

Right but that's missing the key point of combining the solar cell with a rechargeable coin cell, where the lithium battery is intended to be capable of powering the device for several months on end. If the coin cell provides 4 months of operation from some device, maybe a coin + solar could provide indefinite service in the right scenarios?

It would not, because the solar strip offers less power than the device consumes—by a lot. The cell would not really ever charge. You'd need a solar strip that's pretty darn large (by my rough math, 1-2 index cards in area, at least). At that point, how is it a useful form factor? And again, that's assuming the best possible lighting, which is not the case for either a light switch or an air tag.

I suppose you could use a little solar strip to make a button battery last longer? But you're talking about a difference of like 100 days of life versus 102 days of life.

The remote control that came with our Samsung TV has a small solar panel on the back. You just leave it face down when not in use and then never seem to have to charge it via the USB.

If the battery doesn't die, you won't buy another air tag. :-(

Air tags have replaceable batteries

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