Cheers To Finger Power!

Mind you, this is not a “Green” concept and neither does it claim to be “Eco Friendly”. It’s just a helpful solution for a tricky situation. The situation being: you running out of juice on your mobile phone. So what do you do? Remove the battery from the back of the phone; give it a few good turns around your index finger and its gathered enough power to last you a conversation or a safe trip to your charger and electric point.

Designers: Song Teaho & Hyejin Lee

Swing Your Energy - Mobile Phone Battery Charger System by Song Teaho & Hyejin Lee




  • brian t says:

    Let’s do a few “back of the spreadsheet” energy calculations, shall we? (I’m not trying to be a smart alec – this kind of question fascinates me!). I’ll use MKS (metres, kilograms, seconds) units throughout.

    How much energy do you need to generate? My cellphone battery has a capacity of 960 mA.h (milliamp hours) at 3.6V. Let’s say you can make a call with a 10% charge (about 100mAh or 0.1 W.h) at 3.7V -> you need to generate 0.37 W.h or 1332 Joules (W.s) of energy. (J = W.h x 3600 sec/h)

    How much energy can you generate by spinning that battery around? It depends on a number of factors not specified in the piece, so I’ll make some assumptions:
    Mass m = 100g or 0.1 kg
    Rotating Length L = 50mm or 0.1 m
    Inertial constant c, which is a function of the battery’s shape: I’ll say 0.8, which is generous. (This might be the case if the heavy parts are furthest from the centre of rotation.)
    Moment of Inertia I = c x M x L² = 0.8 x 0.1 x 0.1² = 0.0002 kg.m²

    By spinning it up, you’re applying energy from your hand and the result is rotational kinetic energy: E = ½Iω², where ω is the angular velocity. We don’t know what the electrical load will be, but we can calculate how much work is done in keeping the phone spinning at a constant angular velocity (assumning 100% efficiency). Let’s assume you spin the battery at 2 revs per second, so ω = 2 x 2π = 12.57 radians/sec. (The electrical load must be designed to allow spinning by the average person: too much load, you won’t be _able_ to spin it up by hand!)

    Energy required to keep the battery spinning, per revolution: E = ½Iω² = ½ x 0.0002 x 12.57² = 0.0158 Joules per revolution. At this rate, it will take 1332/0.0158 = 84303 revolutions to charge that battery to 10%.

    There may be some errors in my assumptions, but 84303 is several orders of magnitude above the claimed 130 revolutions. This calculation is linear w.r.t. the mass of the battery – double that and the potential energy per spin doubles (if the user can provide it). It follows a square law w.r.t. to the rotating length: double that and the energy quadruples. You can improve the inertial constant by concentrating the mass on the outside. Perhaps you can make a call with less power. Still, if I was evaluating this design as a possible product, I would want to see this sort of calculation in the design documents! 😉

    • VoReason says:

      Thank you, you stated your case clearly, made your assumptions know and logicly and validly reviewed your results.

      This is the difference between an engineer (i.e. someone that makes products a reality) and a designer (someone that sits on their ass and dreams all day). Perhaps I am wrong and you are not an engineer, but someone that enjoys physics but even this persuade is more credible than dreaming up nonsense.

      Hats off to you my good internet patron.

      Well done.

    • eric says:

      isn’t there a typo in the calculations

      e = N = d(Phi)/dt [V]
      e = N * d(Phi)/dt [V] makes more sense if N is the number of turns

      and isnt there another force that practices f=torque on the finger, the part makes the energy. i don’t think the inertia is charging the battery. the coils and the magnets do, don’t they?

    • brian t says:

      Slight correction / clarification to the above. The figure I got, for the number of rotations required, contained another assumption: that the energy was being extracted from the rotation at a particular rate (power), enough to stop the rotation entirely in 1 second (2 rotations) if you stopped spinning it. I think that’s is in the right ballpark, but the actual rate would have to be chosen on the basis of those human factors I mentioned: how much power can the human finger generate this way?

      I imagine the wheel that your finger spins could have gears on its outside (inside the casing, and that could drive a small generator at a higher rotation rate, and generate the electricity. A couple of diodes, maybe a capacitor, that might do it – but an electrical engineer might have a better idea. 8)

    • Argument_Finder says:

      Good analysis, but I don’t think they have a mechanism in the battery for converting the rotational kinetic energy into chemical/electrical energy. You’re not cranking the battery. You’re simply helping the battery consolidate the remaining not-yet-consumed chemical into one place to use for electrical energy.

    • mech says:

      This calculation is pretty much irrelevant. This design does not seem to store energy with its kinetic energy. Think of turning a electric generator, the electricity you produce has nothing to do with the moment of inertia of the rotor. And still what you have calculated "Energy required to keep the battery spinning, per revolution: E = ½Iω² = ½ x 0.0002 x 12.57² = 0.0158 Joules per revolution" is not the nergy required to keep the battery spinning, it is just the kinetic energy of the battery at that speed.
      It seems all the basic physical concepts are misunderstood here.
      The limit in this design would be how much energy can one transfer by his finger, and how much of it can be transferred to the battery.

    • mech says:

      This calculation is pretty much irrelevant. This design does not seem to store energy with its kinetic energy. Think of turning a electric generator, the electricity you produce has nothing to do with the moment of inertia of the rotor. And still what you have calculated “Energy required to keep the battery spinning, per revolution: E = Iω = x 0.0002 x 12.57 = 0.0158 Joules per revolution” is not the nergy required to keep the battery spinning, it is just the kinetic energy of the battery at that speed.
      It seems all the basic physical concepts are misunderstood here.
      The limit in this design would be how much energy can one transfer by his finger, and how much of it can be transferred to the battery.

  • meinv says:

    A very good friend said above

  • Niels says:

    It seems to me that, if you would fill in that hole with more battery, you would have that 10% extra capacity!

    Thank you Brian t for your evaluation of the concept. Your calculations look solid. Now, looking at the concept myself (I know a fair bit about physics, but I am by no means an engineer) I thought that perhaps the winding wheel has some kind of resistance to turning built in – like you get on these wind-up torches, which I presume work by a similar principle. This would create addition friction while spinning the battery at your proposed 2 revolutions per second. Would that result in a higher moment of inertia?

    To VoReason, I kind of see where you are coming from, but do think your views are rather naive. Do you believe the computer you typed that post on would have been as well put together, as small, or as user friendly as it is, if it wast for both designers and engineers working together. Designers pushing boundaries of in what is today possible in engineering terms is a great thing. Designers often ‘dream up’ things that are just not yet possible by todays standards, and sometimes, yes sometimes nonsense. Like trying to defy the laws of physics as above. But that doesn’t mean its was a bad idea.

    In this example, the question was probably something like, why is it that when my battery runs out, that is just it, i’m effed. I shouldn’t have to put up with that right? So introduce a form of winding mechanism. In this instance of swinging it round your finger, it seems that it will prove unworkable due to the laws of physics. But that doesn’t mean a little friction wheel in the corner, so you can race it a few laps around the table or agains a wall, wont be workable, or any other solution that keeps true to the manual recharging idea the project was started out on.

    • brian t says:

      Short answer to the question about inertia: no, it’s a physical property of anything that you want to accelerate, in a straight line or in rotation. You often hear it called “resistance to a change in momentum”. With straight line acceleration, it’s pretty much equivalent to mass – the more mass, the more inertia. With rotating objects, it’s a function of the mass and its distribution relative to the centre of rotation. You can easily test this with a weight on a string: make the string twice as long, it gets 4x harder to spin it – and it has 4x as much energy if it hits something!

      I wouldn’t say this is _totally_ unworkable, just that it won’t be trivial to get the amount of energy you need, in a useful form. There are already hand-cranked phone chargers out there e.g., and you can see it’s a workout, not as easy as spinning a little weight around your finger!

  • R says:

    To the designer: Interesting concept, but according to me not realistic. This is more of a gadget/present that you buy for someone and the other person never using it.

    To VoReason: Yes, you are right. And if everything was made by engineers we would still have huge ugly glasses, squared objects and boring gray colors on everything we use :) Thank god for designers!

  • brian t says:


    I haven’t used a Freeplay radio, but I understand it uses clockwork to produce a smooth flow of power, through gearing to spin a dynamo at a high speed and power the radio. Cranking the handle winds the clockwork, rather than generate electricity directly.

    The problem with adding friction to a device is that that’s an energy loss, and you are trying to extract as much energy as you can from the motion. If there’s a dynamo inside, generating electricity, that is already a dynamic load acting against the motion you apply to it. It won’t spin freely, not when you are extracting energy from it.

    It will take Real Work to charge the battery, which you will feel in your hands, and so I think one key to this idea will be careful testing and tuning of the weight, the natural spin rate of the battery on your finger, and the electrical load (which you’ll feel as physical load). Remember, folks – there is no free lunch! 8)

  • Engineers make products real. Yes, it’s true.
    Designers make products interesting and beautiful and confortable and MARKETABLE. This is also true. I work in the automotive industry, as a designer, but in close colaboration with engineers. Both parties are equally necessary. Neither is more relevant than the other!

    I find it infuriating how some engineers can be so full of themselves, specially in the exact moment when they are being absolutely thick.

    To mr. Voreason: Let me explain to you. Yanko is about original and conceptual designs. This is just a concept! It doesn’t have to work, not right now at least. I know that for some of you engineers, completely unable to draw a cube seeing a perfect rendering may be confusing and make you take ideas for reality. Your work IS to verify the feasibility of the concept and make it possible. This doesn’t make you smarter than anybody!

    DON’T BE AN ASS!!!

    • Part of the dialog here is to point out to designers where they have room to improve, a free critique as it were. Designers for the most part feel that any engineering knowledge will stunt their “vision” and that is horse shit. Designers if they are worth a damn need to be fully aware of physical realities as well as new technologies that allow new ideas to be made real. A badly researched concept just shows the designer has a long way to go to being a pro.

      • Hey Zippy, long time no see.

        Of course a good designer has to know enough of the physical realities at least not to make commit absurds. But it is part of the job of engineers who work side by side with designers to ensure the feasibility of the designs, as it’s part of the designers work to be creative and come up with unique and original solutions. Doing so requires thinking out of the box. I’m pretty sure you know that, for instance, in a brainstorming technique all ideas must be written down for further consideration, even if they look ridiculous at first glance.

        I’d say, by personnal experience, there’s a lot of bad designers, specially those who think themselves artists or something extraordinary, who don’t give a damn for reality but only to their ego trips. On the other hand, a fair share knows their place in the order of things, and are perfectly capable of making a big and significant contribution, technically consistant and original.

        I also have to say, by personnal experience, that the share of engineers who understand the place of the designer is minimal. Also, the share of engineers who think themselves the real and unique core of any development is rather large, not recognizing the importance of all the other parties.

        In my opinion, the core question here is that yanko is about conceptual designs, just like a big brainstorming. For me, everything is allowed, something good may come out of it. Cursing the idea and the designer in a very impolite manner is common here. All I’m asking is for people here to treat concepts like ideas to be further analyzed, treat people like human beings, with respect. Or go to a different web site, about real projects.

        • Canterburry says:

          As a designer you do however have a box to think in, called physics. If the math doesn’t check out, it doesn’t work.

  • designsmog says:

    It’s a nice idea to find a way to get a few last minutes of talktime, but could there be a way of achieving it without having to take your phone apart? I don’t know about you, but mine is quite fiddly…

    How about a battery that recharges when you shake it up and down really hard? (cf or, alternatively, keep the finger hole and the ‘spin’ interaction but move it to the handset itself rather than the battery?

  • trybudi says:

    Victor Assis you’re being an ASS with capital letter. don’t you know i’m working in the automotive industry too? mr VoReason is a designer in an automotive industry too and so is everybody else here

    • Oh god. Why am I an ASS? Because I’m saying designers and engineers are equally indispensable to the process???
      I merely mentioned I work in the automotive industry because it’s a field where designers and engineers colaborate in depth, all the time. I’m not at all saying I’m better than anyone because of that. If you and voReason work in the automotive industry, good for you, you’re not better than anyone because of that either.
      And to your information, a lot of people here are students, and a whole more work in other industry. Sharing opinions free of prejudice and “from the trenches” is good for everybody, specially for the students. Destroying a concept and the designer just because you can’t understand the difference between a concept and a product is not so good.

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  • Dev T Bansal says:

    Seems to be a true good style to be of utility along with being eco-friendly.

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