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Wind-up AA batteries seem like a terrible idea

We’re always looking for new ways to charge our mobile gadgets. Wind, sunlight, fuel cells, you name it. These all seem to rely on an external device pushing power to a rechargeable battery. However, what if the charging mechanism was built into the battery itself? That’s the idea behind this wind-up Charge Batteries.

Yes, I did just say wind-up batteries, and it’s exactly what you’re thinking. Imagine a AA battery that you can wind in order to generate a charge. Do your fingers hurt yet? I’ve had hand-cranked flashlights before, and I remember how long it took to get even a few minutes worth of light. Can you imagine twisting a pair of tiny batteries for extended periods? Something tells me that this concept will never be turned into reality.

1 Comment

  • The question here isn't whether or not this would 'work': clearly, the physics are sound. However, will it work efficiently enough to be of any use to anyone? I'm a little rusty, but let's try and run some numbers (feel free to jump in and correct me if I go wrong).
    Estimating some spring dimensions (0.7mm wire diameter, 13mm outer diameter, 30mm free length) and looking up comparable springs from an online supplier (…) seems to indicate a spring constant (k) of around 0.2 N/mm (200 N/m).
    Hooke's law states that the potential energy stored in a spring is equal to (k(x^2))/2, where k is the spring constant and x is the displacement (distance squeezed/stretched). If we take a wild, optimistic guess that each 360 degree rotation of the battery head produces 1cm of compression in the spring, that means each rotation stores a nicely round (200(.01^2))/2 = 0.01J of potential energy in the spring.
    Let's continue our optimism and say that the mechanism connected to the spring is able to quickly convert that compression into rotation, spinning a tiny generator which converts the kinetic energy into electric potential energy in the battery at, let's say, 80% efficiency. Each twist therefore stores 0.008J, or 0.002 mWh, in the battery.
    A decent NiMH AA battery stores around 2500 mAh. If we assume the cell in the design maintains the same energy density as a normal NiMH battery, at ~30% of the size, we can estimate a fully charged capacity of ~750 mAh. Since AA batteries operate at 1.5V, this gives us a total energy capacity of (1.5 x 750) = 1125 mWh.
    This means that, to fully charge the battery, you'd need to twist the top all the way around (1125/.002) = 562,500 times. Based on my home experiments fondling the top of a normal AA battery, I estimate that it's possible to rotate the top approximately twice per second, which means that fully charging the battery would take (((562 500 / 2) / 60) / 60) / 24 = 3.25 days of continuous twisting. (note: this ignores wrist and elbow fatigue, which I started to experience after only a few hundred twists)
    So more than 3 straight days of twisting one of these suckers, for a third of the power of a standard AA battery. I'd call that unfeasible, unless you had some serious time on your hands and were absolutely desperate for a few milliwatts.

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