Wind-up Double-A Battery - Sound Science packed into an Amazing Design

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 (http://www.leespring.com/browse_catalog.asp?rbunitOfMeasure=...) 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.

Looks like a "cool but useless" idea to me. The spring is not being used to 'really' store energy...

So, you have a crappy (low capacity) rechargeable battery, that is paired with a crappy mechanical charger.

You would be much better served to have good rechargeable batteries, that you put in a good charger (which could be mechanical, if that's what you want).

Oh good, some design students have made a 3D mockup. That sounds like science. The next step in the process is to sit back and rake in the money! Actually producing or testing the thing is just a minor detail.

(Why a battery and not a capacitor?)

Terrible. Complicated, expensive, fragile, and with poor performance. Look at the size of the actual battery! Hint: in normal batteries, the entire battery is the battery.

This is almost as bad as "solar powered batteries."

A battery-charging "shake light" would be more practical, ergonomic, durable, and affordable. You could just buy one, and it would work with your existing batteries.

sound science? i very much doubt this makes sense. if the energy density of springs were comparable to batteries then we would have wind-up teslas on the road (and spring-driven ups in data centres...).

and if the energy is stored in the spring, what's the rechargeable part for? and what kind of generator is efficient, compact and slow? it's a completely improbable mess.

the closest workable solution i can see would be to drop the spring, extend the battery, and have a simple wind-to-charge battery. but that still requires the generator.

Reminds me of the kink springs in Paolo Bacigalupi's novel The Wind-Up Girl.

I was hoping it would have a tiny little flywheel in it or something and be purely mechanical but I guess maybe it needs to charge a "real" battery to keep the flow of power more consistent.

People don't really grok the energy stored in a AA because it seems so innocuous. A rechargeable AA has about 40 joules of PE. That's as much as a handgun bullet. To make that much energy by winding your battery would take all day. Adding a little spring probably makes it worse, not better than just gearing the little generator to be cranked directly.

Wouldn't it be better to replace the spring mechanism with a set of multiplier gears? You should be able to turn the top once and spin the generator a few thousand times, not turn the top once and have it spring-turn the generator once.

This might not be the excellent execution everyone hoped for, but binding both the charger and the battery in a AA format seems innovative to me. Maybe another iteration of the same concept will lead to something interesting

Similar idea but from 3 years ago: http://www.geekalerts.com/wind-up-battery/

That one appears to harness the mechanical energy directly.

Nice idea! Want!

wow loads of nay-sayers. There is nothing hokey about the science, of course it will work. I dont see the issues, it clearly isnt supposed to be a direct replacement for energy hungry batteries, but for low drain, use a lot, maybe even critical applications, like oh, I dont know a transistor radio maybe.