Solar Batteries


[Knut Karlsen] put together a prototype set of solar rechargeable batteries. He always seemed to have batteries laying around on his worktable and figured they might as well be charging. The flexible solar cells were given to him by researchers at the IFE and are rated at 1.8V. He used superglue to secure them to the C cells. A silver conductive pen plus flat wires from a Canon lens connect the solar cells to the battery terminals. The batteries just trickle charge for now, but he’s going to try to build cells with built in charge controllers in the future.

52 thoughts on “Solar Batteries

  1. So inefficient. Probably only 10% of the battery is in direct sunlight at any time. Wouldn’t it be better to mass produce rechargeable batteries, and have one higher powered solar charger for them? And when are you ever going to be leaving all your batteries lying around on a desk in the sun lol, you’d have to move them all around your house to keep up with the sun. Just saying…

  2. how long does it take for these to charge?

    yea, the idea is awesome. but if it takes several days to recharge the battery it seems kind of useless. we already have solar-powered battery rechargers.

  3. Sure it may be slow and not get full sun exposure, but as the author points out – if they’re just sitting around anyway, what have you got to lose?

    This is, of course, assuming you can get cheap enough solar cells..

  4. The Aye’s have it, this idea DESERVES the next level. Maybe a couple of surface mount LEDs to let you know when it’s charging or when it’s charged, this Idea Definitely has a bright future.

  5. I think you all have got the wrong idea. Granted, the panels won’t be that efficient, but they *will* keep the dead battery effect at bay. even if they won’t completely charge the batteries, they will keep them charged.

  6. To increase surface area usage, why not add a ‘scroll’ to the flexible cell, like a roll of film (if anyone remembers those). That way you could orient the cell towards the sun for optimum charging, and if the casing of the ‘roll’ was transparent, you could still charge them when rolled. You’d cut down on the internal volume available for power storage, but many C- and D- cells are casings for AA cells, so there’d be no loss in those cases.

  7. I like the idea… and edz has an interesting concept of “unrolling” the solar array when charging to get a faster recharge; basically don’t glue down all four corners of the flexible cell, just the two where the array is soldered to the battery terminals. this way, when not in a device, it will unroll the array and allow you to charge faster. I’d be interested in just how small a charging circuit you could make… if it could roll into a layer under the array, that would be convenient. think flex circuits and tiny surface mount parts. I wonder if you could also fit the needed circuit on the end of a non-standard shorter battery. so many ideas, so little time.

  8. Genius. As to unrolling the solar array. Remember those snap bracelets that use to e pretty big? Maybe have the same thing with the array. There will need to be a way to tell % of charge and to stop current bleed back. Now electronic devises need to have clear battery covers.

  9. Roll outs:
    Thin film can not yet be durable enough for such application.

    Cell charge/size and controllers:
    The design of each cell size holds a specific amount of power for a specific amount/time of discharge. Additional electrics to prevent trickle drain and to regulate charge would take away from the total storage capacity and thus make these batteries have less capacity; undesirable for length of use applications.

    Perhaps there needs to be a really good article/write up on electronics or solar electronics that can be linked to with any project; all the speculation about already established information can be quite frustrating.

  10. Of course all he needs to do is put the cell on the becnch with two mirros behind it to reflect the sun light onto the back of the cell like this <0

    If you segment the mirror vertically into thin sections you could create a half circle behind the cell with the angles fixed to reflect onto the cell like this (0

  11. This is just silly.

    Solar cells only work if they are entirely illuminated. If there are any shadows on the cell then that cell goes into reverse bias. So for these to work he needs a strong diffuse light or mirrors as another commentor posted.

    And secondly solar cells will rob current when dark, like in a device powering it — thereby discharging the battery!!!. His design has no blocking diode to prevent this.

  12. @tecnik: Cost of wrapping solar panels around batteries is waste when you could just take those same solar panels and stick them to a flat surface that connects to the batteries to charge them.

    If you stick the batteries in a charger, they won’t just be sitting around on your desk anymore AND you’ll have a cleaner desk.

  13. get your self some radioactive waste, mix it up into a paste. Then smear the radioactive goo alover the face of the solar cell. Finally roll up the solar cell into the size of your desired battery. Viola, THis will work during cloudy das too.

  14. How is this a good idea? I remember seeing this kind of thing on a forum years ago and joining in with the bashing.

    First, a solar cell on a cylindrical object is stupid at best, half the cell will always be facing away from the sun. Secondly, batteries are generally inside other objects where they get no light.

    Finally, it is expensive and even ideally the batteries will take days or more to recharge.

    I would consider this only marginally better than buying solar cells and putting them face-down outside.

  15. If the battery were covered with lots of long thin cells each with a schottky diode then it might have more of a chance of utilising the light falling on it, as has been pointed out if part of an individual cell is covered up then the overall power output drops dramatically.

    Also, would [Knut Karlsen]’s batteries self-discharge quicker than the applied solar cells could charge them up? ideally you’d want to apply solar cells to batteries that have an extremely low self-discharge rate, like the Hybrio range for example.

  16. Good Idea, difficult to implement effectively.
    a smd schottky diode would perhaps not interfere with the the contacting requirements of this form factor. A large flashlight reflector would also allow the whole surface to get exposure to sunlight but it would require tracking to be effective, but in the summertime fry your battery :(

  17. I seen a demo at a convention of a small portable solar panel that could not only charge batteries, but could directly power a laptop. It had signal processors and micro controllers to manage levels, and had an on board reserve that could power an average laptop 5 minutes while it gave an alert.

  18. this is a cool idea, and it could save your day if you just can get your application online once again to do it’s job.
    Yes, there are limits to the design, but nothing that couldn’t get counterd in a final design. For example make four independent sections, so one is always in the sun, two partly and one in the dark, decouple those with Schottkydiodes. Two of the mentioned “flaws” solved. I like how people think thats how Cells are charged in future, just throw them on your table, hahaha.
    this will always be a backup solution, just in case.

  19. You people that are saying ‘why not just use a solar powered battery charger’ are missing the point. The advantage of this is that you don’t *have* to remember to put the batteries in a charger. You just leave them lying around. Much easier.

    I’d say its a great idea as long as it takes less than 2 weeks to charge a battery in reasonable conditions. I foresee issues with robustness though; those solar cells are going to get scratched up every time you put them in or take them out of a device.

  20. you guys have to learn to be a little more skeptic. when i was 7 years old, i thought “solar panels = winwinwin!” but after many years of actually playing with/designing with them, I’ve gained a more balanced view of their pros and cons.

    1) when you see a solar panel, learn how to immediately estimate the power it can output. You can start by googling “solar surface area,” etc.

  21. That photo certainly _looks_ fantastic, and it’s certainly a subject that captures all of our imaginations because let’s face it, it’s so groam cool!

    However, as stated in earlier comments, the realities of solar battery charging mean that while it’s an awesome idea it still needs to be refined before it can become something that can be used practically.

  22. Gimmick.

    Less than 8.9% of the solar cell surface is in direct incident with the light. This is simply a “neato” thing and not useful at all. All those flexible cells on a flat plane and pointed at the light source would be at least 600% more efficient.

  23. Consolidated list of problems with this concept:

    * A very small percentage of the solar cells facing the sun at any time.

    * Cost

    * Even the best rechargable batteries will die long before the solar cell making the cell a waste as it’s glued to a dead cell now.

    * Some rechargable batteries, lithium ion in particular, have their total cycle life dependent on the temperature they are stored at. By leaving them in direct sunlight you, probably, shorten their life dramatically.

    * Many types of rechargable batteries, litium ion in particular, require advanced recharging circuits in order to charge without causing damage to the cell. There isn’t any space to put that circuitry and still have the batteries fit in a standard battery slot.

  24. Most of you are putting this project down because it has a couple of problems (cost, etc). There’s a reason it’s called a *prototype*! Cost? When rechargables and solar cells first came out, they were quite costly. Now, you can find a solar-powered calculator for $1. The flexible cells are a relatively new concept. Given time, this project can be a very viable solution for some applications.

  25. Good idea, if only solar cells weren’t expensive as hell and so poor in performance. The already weak solar cell would get only a small part of direct sunlight which makes them nearly useless. Furthermore, the cells that don’t get sunlight will behave as polarized resistors, wasting even more power.
    A flat cell array is still the best solution, both in performance and price.

  26. Thankyou nick for paraphrasing what has allready said 20 times previously. Why bother to paste your comment if your not going to read the rest.

    Hoping that someone will read this far and think “Well, that Nick is really insightfull – the other 50 posters didnt quite sum it up”. Trust me, it doesnt happen.

  27. I wonder if it would be possible (or rather, practicle) to fit a bridge rectifier into a battery case and use it to get power from radio vaves like this:

    As long as the rectifier isn’t put in an electromagneticly sheilded container, it should be able to harvest power, and I’m thinking most portable devices arn’t concerned with sheilding their batteries. From what I’ve read, I think there are some voltage and current regulation issues to deal with, but it might be something for me to work on over the summer once I get the hang of soldering.

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