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A long time ago I learned that electronic wristwatches have a charging device that draws energy from hand movement while the wearer walks. I don't know how much power a wristwatch generates but maybe this sort of tech could be used in modern smartphones to charge them while you're walking. Would the power output of the same generator as in watches be enough to let a smartphone never run out of power? If not, how much more power would be needed? Would such a charger be feasible?

The main point of this charging method is that it should be effortless. With a dynamo generator that definitely outputs much more power you'd absolutely notice getting tired.

user1306322
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    Relevant paper: http://www.cc.gatech.edu/~thad/p/journal/human-powered-wearable-computing.pdf – pjc50 Oct 09 '15 at 13:05
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    A typical cellphone bettery is 1400 mAh x 3.6V = 5 Watt hours. Add up to 50% to get from energy generator to phone battery due to various losses. The typical plastic squeeze lights or crank lights make about 1 Watt when very enthusiastically operated and you will tire in about 5 minutes at that rate. To get say 7.7 Watt hours you need to do that for 7.5 hours. A good quality hand powered alternator can make 56 Watts without too much effort and 10 Watts is beginning to get too enthusiastic for long term use. At 10 Watts you need 45 minutes operation and you'd be veruy tired. At 5 Watts .... – Russell McMahon Oct 09 '15 at 13:23
  • .... it takes 1.5 hours and you'd still know your hand had had a workout. So, doable, with a "real" alternator. An iPad has about 7 x as much battery capacity! – Russell McMahon Oct 09 '15 at 13:25

2 Answers2

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Feasible: yes

Useful: no, because the power needed by a smartphone is orders of magnitude larger than what such a (kinetic) generator generates.

A (non smart) wristwatch needs only very little power, it can run for years on a small button cell. This amount of power can easily be generated by a kinetic generator.

A smartphone has a large battery and needs charging almost daily. Unless you want to violently shake your phone for a couple of hours every day, a kinetic generator will by far not satisfy the power needs of a smartphone.

Bimpelrekkie
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    +1 for "violently shake your phone for a couple of hours every day" :D That could actually be a part of your daily fitness routine... – Arsenal Oct 09 '15 at 13:02
  • Thanks, it could :-) Maybe some company should make a "shake to charge" device so you can charge your phone with that. – Bimpelrekkie Oct 09 '15 at 13:08
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    I've seen "shake-to-charge" flashlights about ten years ago but they were at 30, 40 and 50 cm long and about 1.5 kg. There was a huge magnet inside the tube that was generating power when moving, so I suppose you could repurpose that to charge a phone instead of the lamp's battery. – user1306322 Oct 09 '15 at 13:12
  • @Arsenal, et al.: You might as well plug it into your static bike. http://spectrum.ieee.org/green-tech/conservation/these-exercise-machines-turn-your-sweat-into-electricity – Fizz Oct 09 '15 at 13:35
  • @user1306322 - I had one of those 'shake-to-charge' flashlights. It lit a LED, so about 5mA. It would light the LED for a few x longer than shaking, but IIRC not 10x longer. So maybe 50mA seconds/second of shaking, or 360 seconds of shaking for 5mA hour. AFAIK, the battery is some mobiles could be about 1Ahour, so to fully charge, that would be: 1A/50mA = 20x360seconds, or two hours of shaking for a full 1Ahour charge. (warning, I might be several x wrong :-/). – gbulmer Oct 09 '15 at 13:46
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I haven't seen those chargers in electronic watches, but anyways let's do some rough calculations:

A typical wristwatch uses a CR2032 battery (3V, 230mAh). With that it can run a year (probably more?, don't have a wristwatch). So the average power of a wristwatch would be just 80µW. That is not much at all.

A typical mobile phone lasts 2 days and has a battery of 3.7V with 1600mAh (probably more). So it has an average power of 123mW. Doesn't sound much either, but its roughly 2000 times of what a wristwatch uses.

My calculations were laid out in a way to provoke a positive answer (Yay you can do it!), but it still turned out pretty bad. Now you might imagine that even with the advances in low power energy harvesting it might be impractical to implement a device in a mobile phone big enough to actually make a real difference (half a day or something).

So it seems like a small correction is in order. It seems like my estimation of the wristwatch power is an order of magnitude to high. So it's rather 8µW instead of 80µW.

My phone actually has a 2000mAh battery.

So in the end the phone needs roughly 20000 times as much power. Even the 2000 before were bad, but with another order of magnitude this is going to be completely impractical.

Arsenal
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  • 80uW? MUCH less than that... I've tested a mechanical pocketwatch from 1841 at 10uW and a home-made LED wristwatch at about 2.5 uW (display off) This only strengthens your argument. –  Oct 09 '15 at 13:47
  • @Brian how did you measure the power of the mechanical pocketwatch from 1841? I figured my numbers might be off quite a bit, but like you say, it just makes things worse (for the device, not for my argument) – Arsenal Oct 09 '15 at 14:25
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    Spring winding torque x no. of turns = energy, and 30 hour runtime. It was a fusee movement so the spring torque was constant. –  Oct 09 '15 at 14:37
  • One manufacturer claims ten years for select watches http://electronics.stackexchange.com/q/41582/3552 – sharptooth Oct 09 '15 at 14:47
  • An el-chepo/run-of-the-mill solar-cell watch will easily exceed that. – Fizz Oct 09 '15 at 15:10
  • My experience is that a typical digital watch will run for five to ten years off the much smaller LR44 battery. – Mark Oct 09 '15 at 18:45
  • @Mark it's just getting worse :D Well your numbers end up between 2.5µW to 5µW. Seems like that's the typical value. – Arsenal Oct 09 '15 at 18:49
  • I've got a Movado watch that I've had nearly 10 years running on the same battery. I know someone else who has the same watch and last I knew he'd had it 12+ years on the same battery. – Arielle Lewis Oct 09 '15 at 20:14
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    I guess we can just leave it at that, this is not a contest on who gets the longest runtime out of a battery with a wristwatch. – Arsenal Oct 09 '15 at 20:16