Tuesday, February 24, 2009

Solar Batteries

Via Inhabitat:

Inventor Knut Karlson is working on batteries wrapped in a thin photovoltaic cell- meaning that these batteries charge when simply left out in the sun!

I'm still not sure about the level of "green" associated with using batteries, but rechargable solar batteries sound like a potentially feasible option to me for many of the small power consumers around the house- decentralizing seems to me to be the way to go for relaiability in the case of alternative energy sources. Conter-arguments?


  1. A solar charger with rechargeable batteries is a better idea, because it is equally decentralized but more utilized. Those photovoltaic panels will spend the majority of their lives (in use) in darkness. This is a bad idea for something as expensive a a photovoltaic cell. Whereas if you have many batteries and several chargers, your photocells will always be in use while the batteries are in use. The amount of work that can be done is greater than his system.

  2. I generally agree with zack on this, separate charger and batteries seem cheaper (less photo-cells) and more interchangeable (can charge other batteries).

    Perhaps one situation where these solar batteries would be better would be in cases of limited space/weight. For example if you are traveling (by bike, or hiking)...

  3. Thanks for the thoughts boys.

    Do you know:
    are there existing systems we could use?
    is there an instructable which talks about using solar power in this way? how can it be done cheaply, effectively, and "open-source"?

  4. Just some quick back-of-the-envelope numbers:

    Duracell says their D cells are good for 13 Amp hours, I know a decent rechargeable AA is about 2.2 Amp hours. Nominal for a D cell is 1.5v (NiMH generally run about 1.2, I think), so, generously and back of the envelope-ly, we'll call that 19.5 Watt hours.

    Sunlight on the earth's surface is a kilowatt per square meter directly overhead, practical ends up being about 700 watts per square meter for the brightest 6 hours.

    I'm guessing the exposed area of a D cell is about 40mm x 25mm, or 1/1000th of a square meter (I love it when I pull a number out of my butt and it makes the numbers work easily), which makes the total energy received in bright sun .7 watts.

    19.5/.7 is 27.85 hours at the brightest 6 hours of the day to charge this thing, assuming your solar cells run at 100% efficiency. But they don't, the best labs (ie: people trying to drum up venture capital) have claimed so far is about 40%, I'll toss 25% out there as a number for a solar cell that isn't oriented terribly efficiently and that's going to bang around in devices and get grubby fingers on it and all, so that's over a hundred hours of charging.

    Yeah, the practicality of these things is... uh... low. In fact, I wonder what the storage leakage of NiMH batteries is.

    As for doing it open source, charging lead acid batteries is pretty easy, charging NiCads isn't too much harder, it's just a resistor to limit current flow to make sure you don't charge the battery too fast. But even for NiMH, most off-the-shelf battery chargers are running off of wall warts, my basic consumery AA/AAA charger takes 12 volts 1.5 amps in, so just pop up enough photovoltaic surface area to feed that and you've got a battery charger. And if you really want to roll your own, a battery charger is a common application note from several microprocessor and electronics supply companies (I've read the one from Atmel, for instance).

  5. Thanks for the stats dan, that's helpful to actually have the numbers kicking around.

    I've heard good things about lead acid, (a la airsoft type battery backs), and if the money's around, NiCad's are great. A good example of decentralized charging that I've bumped up against is quite easy, as seen here in the example of a simple gardening light. These are usually a set of 2 or 3 rechargeable double A's, a photovoltaic cell, a TIP 31 Transistor, and a CDS which toggles daytime and nightime behavior, which if shorted, just makes it a battery charger with spare LEDS (since it never does its night-time behavior.)