Batteries, recharged by the Sun

solarbatteries.jpg

Solar panels make even the dullest items look cool! Jason Sahler writes in to tell us of Knut Karlsen’s clever invention:

Karlsen’s SunCat batteries circumvent chargers completely by integrating solar cells within the batteries themselves. To make these prototypes he attached 1.8V flexible photovoltaic cells onto 1.5V NiMH rechargeable batteries and connected them with a conductive silver pen and a few flat wires. The effect is similar to a trickle charger, which slowly charges a battery and can be left attached indefinitely without overcharging.

In other words, they won’t be matching those 15-minute wallwart chargers they bundle with triple-As … and won’t be messing your batteries up after 100 charges, either.

The SunCat Batteries – DIY prototypes [Bareknut via Inhabitat]

About Rob Beschizza

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20 Responses to Batteries, recharged by the Sun

  1. Ito Kagehisa says:

    Jim Kirk, are you sure you aren’t confusing LIon and NiMH? I never use temperature sensors on NiMH, but they are essential on LIon (unless one is fond of unscheduled explosions and fires).

  2. Ito Kagehisa says:

    I certainly agree about the LiIons, Captain! I wasn’t aware of the “heat bump” of NiMHs, though, I’ve always slow-charged them.

    This is interesting & agrees with both of us, I think:

    http://www.powerstream.com/NiMH.htm

    Thanks for helping me understand NiMH!

  3. jimkirk says:

    Hi Ito,

    Yes, NiMH cells heat up when you reach end of charge. Sensing that is a good way to end the charge cycle. That’s also why slowly charging NiMH is a bad idea, they don’t heat up enough to easily sense the increase.

    LiIon shouldn’t heat up when you charge them. They’re very efficient, up to 95% of the energy goes into charging them.

    You are correct that one should monitor the temperature of lithium cells, but that’s a safety issue, not charging. If they do heat up, you have a definite problem.

    I’ve designed products using both chemistries. Once you take care of the safety aspects, which is far from trivial, charging lithium is pretty easy compared to nickel.

  4. dragonfrog says:

    Anon @5

    Now that’s an idea I like – a portable solar cell with a short mini-USB cable. Which will be great until all the vendors supposedly settle on micro-USB, whatever that is.

  5. DMcK says:

    Needn’t be the sun; I would think any light source would work. Can’t speak to this diode problem, though, ’cause I’m ig’nant.

  6. Anonymous says:

    Please NO MORE BAD SOLAR CELL IDEAS.
    Just b/c it’s solar power does not make it environment friendly! Just couple days back, there was a post with solar cells on a cell phone, which would not be so bad, if we didn’t change phones every year. stop producing/consuming waste.

    If you must have solar cells, at least make it generic, so multiple things can be charged by them including cell phones, re-chargable batteries, etc.

  7. adralien says:

    Nice idea, but this would only work for single-cell applications

    Your cells will now all be at different charge levels due to sunlight differences, and the first time you put them in series for a discharge you will over-discharge the cell with the lowest charge state which will kill it, and possibly cause it to leak.

  8. dculberson says:

    Yeah, there’s no reason to say those cells don’t already have blocking diodes. Plus maybe it was just a guy having fun with a concept, rather than necessarily a serious proposal for something to be manufactured.

  9. winkybb says:

    Dumb idea, like a lot of these small-scale “green” solutions. Effective and efficient energy solutions need to be at an industrial scale. Pool the dollars that are being frittered away on this and other small, inefficent (physically and econmically) off-grid ideas and build some REALLY BIG solar farms.

    Oh, and also, last time I checked all my batteries were inside devices and not out getting a tan.

  10. shanealeslie says:

    Thanks for all the engineering/science comments everyone, I found them very informative.

  11. jimkirk says:

    A blocking diode is nice, but not always necessary. Enough solar cells in series will have enough cumulative forward drop so that the forward biased junctions of the cells won’t conduct significantly. There is a shunt resistance to consider, that’s probably a pretty low current path.

    That said, most battery manufacturers don’t recommend long term trickle charging of NiMH cells, and if you do want to trickle charge, you have to accurately characterize your batteries. The variability of available charging current in this application would suggest against this. (Knut does mention a possible second version that would check when full, and hopefully stop charging.)

    Termination of charge is a tricky thing with NiMH batteries, usually based on sensing a slight DECREASE of voltage when fully charged, or a rapid increase in the rate of cell temperature rise, so fast (but not too fast) charging is recommended.

    I designed and built a similar device with a LiIon cell. It has multiple over-charge and over-discharge protection, a thermal barrier between solar cell and battery and a little 5 volt boost power supply outputting to a USB connector. The whole thing is about a quarter inch think.

    Adralien is quite right about using batteries of varying states of charge. Over-discharging, which is quite likely with batteries of different states of charge, will permanently damage cells.

    As far as elevated temperature, a shiny reflective battery in the sun is probably better off thermally than one surrounded by something designed to absorb energy, even considering the small amount of energy converted to electricity.

    Plus, the self-discharge current increases with temperature, so depending on the solar cells and where you place the batteries, they might not even overcome the increased self-discharge. Putting then on the dashboard of a car in summer is probably a bad idea.

  12. Nelson.C says:

    The rapid chargers ruin batteries? I didn’t know that. Rats.

  13. mappo says:

    Batteries…direct sunlight…why didn’t I think of that? Oh yeah, because that’s a HORRIBLE idea, that’s why!

  14. Alpinwolf says:

    NiMH get hot too, I think that’s usually what NiMH chargers sense to stop charging. But perhaps I lie…

    Also, if one ignores the rotten cultural slur for a moment, this reminds me of one of those Polish Inventions jokes of a solar-powered flashlight.

    (Even when I was, like, 8, I understood the jokes, but found the cultural element confusing and un-funny. Never knew why Poles were picked on. Maybe that part’s regional.)

    Anyways, this idea seems about as well planned.

  15. Anonymous says:

    Great idea, wrap around solar cells on batteries are a perfect solution to our ecological problems. As long as the device youre powering is made entirely of glass. And is floating in outer space. Between the two stars of a binary system.

    Congratulations Knut Karlsen though, for coming up with an idea even more retarded than that solar powered phone from a few days ago.

  16. semiotix says:

    Where the hell am I supposed to find sunlight in this day and age? It’s not 1973, I don’t need the batteries for my portable 8-track player. JEEZ!

  17. shanealeslie says:

    I’ve heard a couple of people mention that batteries + direct sunlight would be bad, but wouldn’t the solar cells conversion of the energy from the sun into charge (at least somewhat) negate the overheating? Any engineers have any idea about the math behind this?

  18. Telecustard says:

    Sad part is, this isn’t even a new idea. I saw “D” cells with integral solar panels in the “What’s New” section of Popular Science over 20 years ago. They claimed that these would be ideal for backpackers. I never saw them again after that, so perhaps as many have said here, they aren’t so terrific. I just wish that people would stop inventing “new” things that are just ripoffs of other people’s work. Like the guys at MIT with their “wireless power” which was ripping off Nikola Tesla shamelessly.

  19. teuthis says:

    Without a diode they will discharge into the solar cell when in darkness. Much more quickly than they can charge.

  20. Ito Kagehisa says:

    A shaded solar cell connected to a battery will draw electricity out of the battery. A “blocking diode” at the battery will prevent this. A diode is a “one way gate” for electricity.

    In a solar panel, multiple solar cells are connected together. Each individual solar cell needs a blocking diode if any part of the panel will ever be shaded, because a solar panel without blocking diodes on each cell will produce no power if it is half in sun, half in shade. The shaded cells in the panel will consume the energy the illuminated cells produce, and nothing will reach the battery.

    Most large solar panels sold today include blocking diodes on each cell, and modern battery charge controllers always have integral blocking diodes.

    Some technologies produce partial output when an individual cell is partly illuminated, others produce none. Because the experimentor has not given any information about the thin-film solar cells that were used, I can’t say much more, very sorry.

    The obvious way to illuminate the entire solar cell surface on these devices would be to place each one in a parabolic reflector aimed at the sun – a cheap stainless bowl or wok should do, perhaps lined with foil. Unfortunately, as others have noted, temperature has effects on both batteries and solar cells and has to be managed carefully.

    A Lacrosse programmable charger and a 12 volt backpacker panel would probably be better, but personally I like to see people experimenting. One learns as much from failure as success.

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