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Monday, September 18, 2017

Wireless at-distance charging may be here - and not how you were expecting

There are a number of wireless power companies out there claiming all sorts of amazing specifications of their seemingly perpetually "soon to be released" systems - 15 feet range, faster than a wire charging, 10's of devices, cheap, safe, efficient - yet few can answer detailed questions as to the physics, or the release dates, or the safety, just vague future promises that all will be revealed. One exception was Disney who went into extensive detail about their "quasistatic cavity response" method which they demonstrated worked, but clearly was utterly impractical (hence the detailed reveal).

Well, things may have changed today, with the public announcement by Pi of their charging solution. If you want to watch the demo they gave today, you can watch it here, or read some of the other press release material here, and articles from Techcrunch or DigitalTrends on it.

Above is a picture from their website as to typical usage, and it's basically a cone shaped transmitter that looks a bit larger than an Amazon Echo or Google Home in height, and looks to charge devices within a short range of it, in any orientation. If you go to the "Tech" page there you can see a representative breakdown of what's in the box. It uses magnetic resonance, controlling magnetic fields to transfer power in the same manner that Qi does - you may have heard of that recently, as Apple announced last Tuesday that this form of wireless charging will be in the iPhone 8 and iPhone X (it's already in most Samsung high end phones).

The limitation to Qi has previously been that a charging pad is required, and the phone has to be within a centimeter or so of the pad, and well aligned. This makes the phone almost unusable during that charging time, and limits how many devices you can charge to the size of your pad. 

Pi's approach seems to get around most of those limitations by being able to 'beamform' the magnetic field, their claim is that the equations to do so have been simplified so that they can be done on the processors available in consumer goods. The device itself seems to have multiple coils, at least one of which is at 90 degrees to the others, and is likely needed to be able to direct the fields as required - it also is what likely sets the cone shape of the transmitter.

They claim about 20W total power output from this device, but that it can go higher with a larger transmitter. Qi calls for 5W in its early version and 15W in the latest. Pi state up to 4 devices can charge at full rate and then it starts to slow after, which would point to them using the 5W rate. Their website says 10W per device, so what rates are used are still a little unclear. Still, in theory 5W will charge your phone in an hour, although in practice it will be longer than that - but it's a reasonable amount of power.

The downside to this device? The working distance is about 30cm, so not the huge distances we've been promised by other companies, but in terms of making the base technology better, it's a 10x improvement so it's impressive work. It's unfortunate that they may receive criticism because expectations have been set so high by companies that I don't believe will ever deliver their initial claims.

It's a usually-safe, well known technology, and unlike uBeam or Energous, Pi have actually published some aspects of their approach, such as here, so you can actually begin to analyze what they've done. It's not complete work, absolute numbers are not included, and it's not peer-reviewed, but I can follow their approach and there are no huge red flags appearing.

Now I've been critical of uBeam and Energous for 'showing a charge light coming on' or claims of charge rates incompatible with FCC regulations, so why am I not so critical of Pi? Quite simply, it's because they're not claiming anything that isn't readily believable based on the physics of what they are doing, and they are explaining what they are doing. It builds on an already proven (though limited) technology, and uses some smart ideas to move it forward and make it more practical. 

Now, I do want to see more - I want to know the efficiency under various conditions, we may find it's hideously inefficient, though I expect it's well into double digit %. An efficiency at 10 to 20% is decent for at-distance wireless, but poor compared to wired and has societal implications at-scale. I want to test safety and what happens when you start to put metals in the path randomly. I'd love to have one and test it and take it apart, but at first glance I'm not seeing any claims that aren't realistic.

Release is slated for next year, and given the demo seen, they're closer to production equivalent devices than prototype, and unlike other companies who seem to think you can go from prototype to consumer sales in weeks, this is a more realistic timeline.

So let's summarize what we have:
  • Based on a proven power transfer technology
  • Legal and to my knowledge safe
  • Using a smart method to overcome existing limitations
  • Works with inbuilt receivers in modern phones (cases for older ones)
  • A transmitter priced for consumers (under $200)
  • Multi-device charging
  • Reasonable charge rates (5W and up)
  • Can use the device in any orientation while it charges
  • Works internationally
  • Modest distance, 30cm
  • Unknown efficiency ranges (estimated below)
  • Timing near perfect to match Apple's AirPower release

It's an easy, minimal effort for a user, they don't have to change their phone if it's new enough (no need to integrate with the phone manufacturer is a huge plus), cost isn't prohibitive, and they can piggy-back on Apple's AirPower marketing. It also leverages a massive prior investment in Qi by many other companies. If this is as good as it is presented, I'm going to say that in my opinion this is as good as at-distance wireless charging is going to get for some time. You can complain about it only being 30cm, I actually don't know if it's going to be enough, but I can see it working for some office desks, hotel, and home locations. They do mention larger ranges from bigger transmitters (don't expect 15 feet though).

Oh, and apparently they've done this on just $3.5 million of investment. Tip-of-the-hat to the Pi team, and I'm looking forward to seeing more detail on this. 

I think uBeam and Energous are going to have a harder time explaining their value proposition after today.

(Also, if you sign up early enough on their website, it's $50 off the transmitter when it comes out).

Update Sept 19th: Some people have been asking why there are no efficiency numbers given. First of all, they won't be great, as in "90%+", and second it's a complex thing to explain to a lay-audience when your efficiency and charge rate are dependent on distance and orientation as it's not a single number. The paper here gives some indication, as it shows charging time (not efficiency) vs distance and orientation. If input power remains constant, and the results here are correct, you have a 50% efficiency reduction from baseline at 30cm. The orientation data doesn't make it clear what the baseline they use is, but seems to be from about the same to up to a 66% loss (take this number with a pinch of salt, that graph isn't the best). This data shows Qi as around 60% efficient as the baseline, so that would indicate Pi goes anywhere from about 60% efficient down to 10%, I'm going to hand-wave estimate a 20 to 25% in most typical use cases. Great for at-distance wireless, not good compared to a wire. (Caution, these are very hand-wavy calculations)

To put into context, imagine they get 25% market penetration on 1 billion Qi enabled phones globally. At 250 million devices, if they all charge once a day and evenly spread and given a 120 minute charge time, that's ~21 million people charging at 5 Watts, but if 25% efficient using 20W, so it's around  0.5 GW of extra global generation capacity. At around $3,500 per kW to construct that's $1.5 billion in new power stations, and at 12c /kWh around $1 million a day burned as heat.

Update Sept 21st: Just a few thoughts. The term "beam forming" is a bit much I think for having two to three coils, and it's not really a beam, it's the magnetic field. Likely "field shaping" or "field biasing". I expect they've tried to balance the number of coils and the associated electronics with cost, efficiency etc and come out with this as what they think is the right mix (<$200 is pretty compelling as a price point).

Update 1/1/18: Quite a few people are saying that Pi charging is a fraud because they've seen 6.78MHz noted for the system while Qi standard Apple uses is nearer 100 kHz. It looks to me like the founder used 6.78 MHZ Rezance standard during his thesis work and then switched to 100 kHz for this product. They clearly state "Qi standard" in articles, but don't mention the frequency in any place I can find. I'm looking for that and will post a link as soon as I can.

Final note: As every time I post something on wireless charging I get accused of writing for financial gain in trying to drive a stock price down, or pump one up, I'll once again note that I have no financial stake here, either long or short, nor am I being paid by any wireless power company. So many people find it hard to understand why I write these posts if not for money, and unfortunately there's nothing I can do to persuade them that other motivations do exist.


  1. If you are a consumer electronics company looking to go Qi or wait on the promise of uBeam or Energous its obvious you join the Qi crowd given this technology.

  2. Hi Paul -

    Been following your blog for some time, have found the articles interesting and informative.

    Given that you seem very knowledgeable about the practical limitations of current technology and are clearly deft with back-of-the-envelope calculations, I'm curious as to what attracted you to joining UBeam?

    Did you expect to see (or develop) a novel technology that would overcome the limitations in practicality you've identified? An opportunity to work with talented and cool people, even if in a futile effort? Or maybe simply a well-compensated job opportunity that met your career/life needs at the time?

    No wrong answer here, just curious as to what initially brought you in, and what ultimately convinced you that the attraction was not valid.

    1. Thanks for your comment and reading the blog.

      One of my earliest posts covers why I joined. It goes into detail on that


      The short summary - it was a very, very challenging technical problem, and the tech people I was working with (Marc and Matt) were some of the best I'd worked with.

      The rest of my blog kinda covers the rest - read from the beginning and it should be clear. Beyond what you can read here, I'd be getting into NDA-breaking territory and I won't do that.

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  4. One issue that Pi does not raise is the presence of metal in the vicinity of the charging dock. On the other hand, I think that they found an elegant solution for a multi-body magnetic charging solution (just google the paper 'Wireless Power Hotspot that Charges All of Your Devices' from 2015 for the math behind their tech). But as you point out, efficiency does suffer. I think that stray metal will also cause issues -- might create no charge zones.
    At $200, the solution is expensive -- Qi xmit pads from China are $2 (who needs certification?).

    1. Just noticed that you refer to the same paper. My bad.

    2. No problem, I appreciate any constructive comment here.

      Yes, it's more expensive at $200 than a charge pad ($2 is a bit low, let's say $10), but it does charge up to 4 devices and doesn't need alignment in the same way. So - multi charging capacity, no alignment, and no thinking as you drop it there in return for paying extra cash for the device and wasted electricity on inefficiency?

      Wireless charging at-a-distance will inevitably come with some downsides. Have they got the balance right? I think, if they deliver what they show here, that they've got as good a shot as anyone will. I do not believe the physics of any known existing tech supports long distance (>1m), efficient, safe, multi-watt charging.

      What about metal in the path? As mentioned, I'd like to see how it deals with that. Can't say anything beyond that right now.

      This is going to be a good test of how much consumers really want at-distance wireless charging. The market can decide on this one.

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  6. This approach is very impressive - note that since the guy is an MIT PhD, his dissertation is available freely on DSpace ("Orientation-indepedent wireless charging of multiple mobile devices at a distance"). In it he has a few efficiency analyses that may be of interest and seem very reasonable.