This has long been a nagging idea: many watches utilize gears to harness the movement of a person’s wrist to power the watch. Now, I realize that this is old, analog technology, put it speaks to a conceptual idea: Would it be possible to capture whatever kinetic energy is produced by the movement of a cell phone being constantly maneuvered by its user to impact battery life? Even if you could only slow the battery’s drain, I would think that an innovation which did this would be hugely impactful.
There are already several devices on the market that do exactly that.
Most of the ones that I have seen have a battery pack plus a kinetic charger. I’ve never tried to use one personally, but from the reviews that I have seen, the battery pack works exactly as advertised.
The kinetic charger, at least in the reviews that I have seen, does not work anywhere near as well as advertised. But it does at least provide some charging, so it’s not completely worthless.
On a related note, there was a lot of work being done in the field of energy harvesting/energy scavenging ten to twenty years ago. I think interest in this area has waned, though. A lot of it turned out to be impractical or inefficient.
That confirms the suspicion I had when I read the OP: cell phones use far more energy than watches.
Here’s a typical watch battery. 1.55 volts, 160 mAh, so total energy 893 J. Assume this lasts one year, so daily energy consumption is 2.45 J.
The iPhone 11’s battery is 3.79 volts, 2942 mAh, for a total energy content of 40,140 J. Assume this lasts one day with “normal” usage, so daily energy consumption is 40,140 J. That’s about 16,384 times the daily energy demand of the watch.
OP also appears to be inquiring about a kinetic charger that’s integral to the phone, in the same way that kinetic chargers are integral to watches. I think phones aren’t typically subjected to the same usable accelerations/displacements that a watch is. Put the watch on your wrist, and the simple act of walking (swinging your arms back and forth) puts the watch’s kinetic charger to work. But your cell phone spends most of its time…in your pocket? Purse? Those locations experience smaller accelerations/displacements than your wrist, so there’s less opportunity for extracting useful work for electrical power generation.
Given the vastly different energy demands of a phone and a wristwatch, and the very different opportunities for power production for a wrist-mounted watch and a pocket-stored phone, I suspect you’d need an impractically large and heavy kinetic charger to do anything even remotely useful for your phone.
If your concern is for a source of electricity to power cell phones when the power goes out for long periods of time you might look at hand crank generators.
Actually, more practically, I was just thinking it could slow the battery drain. So, you fully charge your phone by plugging it in, but it captures however much energy it can from being moved regularly, thereby keeping the charge for longer.
But the replies seem to suggest that this is not practical, since you’d have to wear an entire additional setup, rather than just add some technology to the phone itself.
Harnessing the energy of small bending motions
*New device could provide electrical power source from walking and other ambient motions *
David L. Chandler | MIT News Office
January 6, 2016
For many applications such as biomedical, mechanical, or environmental monitoring devices, harnessing the energy of small motions could provide a small but virtually unlimited power supply. While a number of approaches have been attempted, researchers at MIT have now developed a completely new method based on electrochemical principles, which could be capable of harvesting energy from a broader range of natural motions and activities, including walking.
The new system, based on the slight bending of a sandwich of metal and polymer sheets, is described in the journal Nature Communications, in a paper by MIT professor Ju Li, graduate students Sangtae Kim and Soon Ju Choi, and four others.
Most previously designed devices for harnessing small motions have been based on the triboelectric effect (essentially friction, like rubbing a balloon against a wool sweater) or piezoelectrics (crystals that produce a small voltage when bent or compressed). These work well for high-frequency sources of motion such as those produced by the vibrations of machinery. But for typical human-scale motions such as walking or exercising, such systems have limits.
[…]
[T]he new system uses technology similar to that in lithium ion batteries, so it could likely be produced inexpensively at large scale, Li says. In addition, these devices would be inherently flexible, making them more compatible with wearable technology and less likely to break under mechanical stress.
The Nature Communications article (full text!) is here.
Joe Paradiso at MIT has done a lot of work on electric energy generated and captured from footwear. See, eg, Shenck, N.S., Paradiso, J.A., “Energy Scavenging with Shoe-Mounted Piezoelectrics,” IEEE Micro, Vol. 21, No. 3, May-June 2001, pp. 30-42.
Hmmm. Let’s suppose we came up with some efficient way to harvest 40,140J of kinetic energy from my walking around. Given that there is no energy for nothing, what would a 40,140J drag on my gait feel like? Would I get tired out more quickly, or is that amount of energy negligible to the total amount I have available? I assume it takes a bit more than 40,000 Joules to drag my 230 pound butt around.
From hiking forums, it seems like the one kinetic way of harvesting enough energy would be build something into the hiking boot, this would take the full weight of the person to produce power and was the only practical way to make enough usable power some have found - and that power would go into a battery. However the old hiking adage 1 pound on one’s feet is worth 5 in the pack, means increasing footwear weight is undesirable. Other kinetic devices just wouldn’t produce enough power, and that is with people walking all day long.
Your phone charges with a wall charger in about an hour. Suppose we want to do the same, i.e. charge your phone while walking for an hour. You walk about three miles in that time, or 4828 meters. 40,140 joules divided by 4828 meters is 8.3 N, the force gravity exerts on 0.85 kg. Basically about 1.9 pounds. This assumes a 100% efficient charger, with no thermal losses in the battery while charging, either.
So imagine some asshole walking behind you, constantly tugging on a bungee attached to your belt, and you get the idea.
Alternatively, it would be like climbing 40,000 J / 9.8 m/s^2 / 104 kg = 39 meters (128 feet) of stairs. Not really that bad, especially if spread over a day.