

Wind-up Double-A Battery - Sound Science packed into an Amazing Design - jakejake1
http://quintessentialcool.com/home/wind-up-double-a-battery/

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dandelany
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=...](http://www.leespring.com/browse_catalog.asp?rbunitOfMeasure=Metric&springType=C&partnum=&UnitOfMeasure=Metric&specsCriteria=%28fl_dimension+between+0.984+and+1.378%29+and+%28size_dimension+between+0.02+and+0.035%29+and+%28od_front_dimension+between+0.461+and+0.563%29+and+%28%28int%28cCUR%28iif%28isnull%28solid_ht_dimesion%29%2C0%2Csolid_ht_dimesion%29%29*+1000%29%2F1000%29%3C%3D0.787%29+and+%28%28int%28cCUR%28iif%28isnull%28to_work_over_rod%29%2C0%2Cto_work_over_rod%29%29*+1000%29%2F1000%29%3E%3D0.433%29+and+%28InStr%281%2C%27MW+++%2C+SS+++%2C+OTCS+%2C+SS316%2C+17-7+PH%2C+ELGILOY%2C+ULTEM%27%2Cmid%28%5Bmaterial_alloy%5D%2C1%2C5%29%2C1%29%3E0%29&subType=&isMIL=&searchHistory=2))
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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dustyleary
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).

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jrockway
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?)

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schiffern
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.

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andrewcooke
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.

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jameskilton
Read it again, you missed half of the process.

You twist to compress the spring. This spring then releases, causing a
recharge generator to rotate and _that_ recharges the actual battery.

Rather ingenious I say.

~~~
andrewcooke
so what's the spring for? why not just rotate the generator directly (as i
suggested)? or, if the spring is to store energy, what is the battery for? it
makes no sense.

~~~
nrp
It's to take up the rest of the space in the 3D render.

In seriousness, it is an interesting idea by some designers who presumably
either didn't understand or didn't care why their imagined implementation was
silly or wouldn't work.

~~~
tesseract
They could have at least drawn a torsion spring rather than a compression
spring, and a generator with a more realistic aspect ratio, and I think that
would have given them more space for the actual battery.

Overall the idea seems like "sound science" (engineering really) in the sense
that there's no reason it would be impossible or even difficult to build. It
just... has no practical value at all.

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

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JamisonM
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.

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noonespecial
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.

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joezydeco
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.

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Zarathust
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

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

That one appears to harness the mechanical energy directly.

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Codhisattva
Nice idea! Want!

~~~
felixfurtak
Bad science. This is unlikely to actually work.

~~~
Codhisattva
Oh really? How many milliamps do you think you need?

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andrewbg
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.

