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A certain product (link: Metro, somewhat safe for work) is currently trending on social media, that allows men to convert the kinetic energy of their more vigorous personal activities into electrical energy to recharge their phone.

I've had kinetic watches before, which re-charge by the day-to-day movement through wear; all of which have needed a few extra shakes periodically to keep them going- so it seems dubious to me that the band could really generate enough energy to charge my phone, which must require substantially more power...

Would/does the W*nkBand (product site: nsfw) work?

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    $\begingroup$ Disclaimer: I'm not the downvoter. I don't know if this is an engineering question per se. It could be answered by studies and tests, but it's more of a Skeptics question than an Engineering question. $\endgroup$
    – HDE 226868
    Feb 28, 2015 at 18:46
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    $\begingroup$ Given that the device does not exist and (very)probably never will the mechanism of operation is moot. Maximum energy efficiency and power from hand operated energy harvesters is obtained from rotary crank type units. Low quality plastic squeeze or crank give up to about 1 Watt max and even 5 minutes operation is tiring. A well designed hand crank is near effortless at 1W, easy to use at 5W, very busy at 10W and absolutely frenetic at 20W. .... $\endgroup$
    – Russell McMahon
    Mar 1, 2015 at 6:07
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    $\begingroup$ .... At 5W you'd generate the about 5.5 Watt hour in an eg iPhone battery in somewhat over 1 hour at 100% and realistically 2+ hours. 5 minutes operation at 5W would probably allow a modest call or some texts. $\endgroup$
    – Russell McMahon
    Mar 1, 2015 at 6:08
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    $\begingroup$ @Russell McMahon : you should make that an answer; although I'm not convinced a crank is analogous to a wrist band in motion.... $\endgroup$
    – thomasmichaelwallace
    Mar 1, 2015 at 8:04
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    $\begingroup$ @thomasmichaelwallace I wasn't suggesting the two were the same (although the linear projection of rotary uniform motion is SHM) but just observing what is most effective. For hand-generated energy for lighting or water purification or battery charging ... the crank allows muscles to work in ~ SHM, provides smooth motion and allows flywheel effect. I've worked with exercise bikes and am involved with developing country lighting. I tested a properly implemented hand crank system to see how it compared with alternatives. Generally foot power is better unless there is a reason not to use it. $\endgroup$
    – Russell McMahon
    Mar 1, 2015 at 11:42

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Because this question has proven a little more derisive than I had anticipated (where's that engineering sense of fun/humour?)- I thought I'd attempt an answer as well, if only to prove it's an engineering question which can be answered using the engineering method; although hopefully someone using something more than Google will be able to chip in with a much more informed answer...

I'm definitely not an electronic engineer- but as I see it, sooner or later this question comes down to:

$$E = CK$$

Where

  • $E$ is electric energy stored/used by the phone
  • $C$ is a constant determined by the efficiencies of converting, storing and transfering the original kinetic energy to the electric energy.
  • $K$ is the original kinetic energy generated.

It turns out that kinetic energy harvesting is actually a major field of research and development (especially with the dawning advent of wearable tech) This article states that intentionally shaking an object (which, we could call analogous with the band's usage) could generate something in the region of 3 milliwatts.

According to another forum (which I admit isn't the best reference); there are very few losses in the conversion of kinetic energy to electricity- so around about 95% of the 3 mW will be converted.

A lithium ion battery has around 80-90% charge/discharge efficiency; which means that 90% of 95% of the original 3 mW will make it to the phone.

This means that the energy given to the phone by the band is:

$$ E = (95 \% \times 90 \% ) \times 3\text{mW} = 2.565 \text{mW} $$

Apparently an iPhone battery holds around 5 watt hours.

This means that the band would require:

$$ \frac{ 1 \text{W} \times 5 \text{hours} } { 2.565 \text{mW} } = 81 \text{days} $$

However, I'm guessing it was never designed to be the primary power source- and looking at the same reddit post quoted for the battery capacity- it seems that the minimum delivery to keep your phone going (i.e. standby power usage) is 21.8 mW; which means that there's a 7:1 ratio of generation to usage time (let alone the suggestion of the same article that 1W is required to register as 'charging').

From researching the answer to this question- I'm beginning to think it's just a hoax for a bit of marketing exposure- but I'll be the first to admit that electricity and its generation is far beyond my training as a structural engineer; so maybe someone will correct me?

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