The newest iPhone — the iPhone 8, announced this week—features wireless charging, meaning you can charge it by laying it on a special pad, without plugging it in.

It will likely serve as an introduction to wireless charging to a mass audience. But it is not going to be a niche feature for long. It has matured, standards are being set, and the technology is on the verge of mass adoption.

What follows is a beginner's primer on wireless charging — the competing technologies, where they stand today, and when you can expect to see them. (In a follow-up post, I focus on wireless charging for electric vehicles.)

Electricity is amazing. You spin a coil of wire through a magnetic field and next thing you know everyone with access to an outlet has clean, quiet, instantaneous, and virtually unlimited power at their fingertips, whenever they need it. Wild.

It does have a couple of drawbacks, though.

One, mobile electronic devices — lawnmowers, phones, electric vehicles (EVs), drones, what have you — have big batteries and limited range. You have to remember to plug them in periodically, or replace the batteries, or you're screwed. This "range anxiety" has been particularly troublesome for electric vehicles, as people are naturally concerned about being stranded, but it also serves as a background constraint on almost all portable electronics.

Two, electronic devices that aren't mobile have to be plugged in, leading to a tangle of wires behind every computer, TV, and kitchen appliance. Plugging things in may not seem like that much of a hassle, but it shapes the way we live with electronic devices in subtle and pervasive ways.

Imagine, then: What if everything electronic could be charging all the time, without wires? What if wireless chargers were built into tables, counters, desks, roads, and parking spots, continually recharging the electronic devices around them? What if electric power became ambient?

That scenario is not science fiction. There are wireless charging technologies on the market, more coming in the next year or two, and more still proving their viability in laboratories. Freedom from wires is no longer a pipe dream.

But there's a long road between the lab and commercial viability — and a longer road still to ubiquity. A wireless future is visible over the horizon, but it is by no means inevitable.

All right, let's dive in!

Wireless electricity is not new, but it's about to bust out of obscurity

Wireless power transmission (WPT) — as improbable and borderline magical as it seems — has been around for a long time.

Way back in 1831, Michael Faraday discovered electromagnetic induction: Put crudely, a current running through a wire creates a magnetic field that induces current in nearby wires.

In the 1890s, badass Serbian inventor Nikola Tesla built an experimental laboratory in Colorado Springs where he demonstrated the effect at a distance, with his "magnifying transmitter." He wound a lot of wire into a coil (now known as a Tesla coil) and ran a lot of current through it; a lot of electricity subsequently jumped to nearby coils. It was pretty cool.

Wireless charging has been used in limited applications ever since, but it's generally been too finicky, expensive, and inefficient to catch on for wide commercial use.

That is changing, though. Three big sources of demand are pulling wireless charging forward: the spread of consumer and wearable electronic devices, the electrification of vehicles, and the automation of industrial processes. (In this post I'm mostly going to focus on the first.)

The demand pull has created a surge of interest and activity. To get a sense of the range of stuff going on, it helps to understand how wireless charging works.

Wireless power works like wireless information, only more intense

At the most basic level, WPT involves a transmitter of some kind and a receiver of some kind. The transmitter converts electrical energy into a time-varying electromagnetic field; the receiver converts it back into electricity.

Sharp-eyed readers will note that this is roughly the same way wireless information transfer works. TV broadcasting, radio, wifi — they all use the same electromagnetic fields and waves. In all those cases, some energy is transmitted as well, but only a tiny amount, enough to preserve the integrity of the signal. The signal is the point.

With WPT, the amount of energy that reaches the receiver is the point. That's why its range is much more limited than wifi or radio; it's difficult to transmit energy very far without it diffusing.

So let's take a tour of WPT technologies — the ones already available commercially and the ones struggling to make the leap from lab to market.

Near-field power transmission

WPT divides into two big buckets, loosely classified according to range of transmission: near-field (non-radiative) and far-field (radiative).

Near-field means within roughly one wavelength of the transmitter, a distance that will vary based on the size and structure of the transmitter. Transmitters small enough to fit in cellphone charging pads usually have a range of around a centimeter, which is why phones have to sit directly on them. Larger transmitters, as in EV chargers, can make it several inches.

Within one wavelength, magnetic and electric fields remain separate. Consequently, there are two forms of near-field transmission, inductive coupling (which uses magnetic fields) and capacitive coupling (which uses electrical fields).

1) Inductive coupling

With inductive coupling, power is transferred between two coils of wire via a magnetic field.

Almost all currently available wireless charging products use inductive coupling. That's what your electronic toothbrush uses. And it's what all the new wireless cellphone charging pads use.

For most current phones, you also have to buy some kind of case to serve as a receiver, but in many new phones (e.g., the Samsung Galaxy S7) the receiver is being built in.

The process of getting wireless charging into phones has been slowed by confusion over standards. There are currently two separate standards governing inductive coupling, the Wireless Power Consortium's "Qi" standard and another from the Power Matters Alliance, which has since merged with the AirFuel Alliance, and … you know what? Standards talk is boring. More here if you're interested.

Inductive coupling for phone charging has its critics. It's still slower than charging by cable, and less efficient, and it generates more heat, which reduces battery life. Worst of all, you have to position your phone precisely on a pad (which has to be plugged in!) and leave it there, which a little more convenient than plugging it in directly, but only a little.

Nonetheless, products are beginning to catch on. Ikea already has a line of furniture with chargers built in.



Inductive coupling is also behind the only wireless electric vehicle (EV) charging system currently available on the consumer market: Plugless Power. Virginia-based Evatran began developing the product in 2009, sold its first products to Google in 2011, and began selling to consumers in 2014.

I'll have more to say about Plugless Power in my next post. Suffice to say, even given the limitations of inductive coupling, wireless charging has the potential to change transportation systems in all sorts of interesting ways.

2) Capacitive coupling

The other kind of near-field transmission is capacitive coupling, whereby power is transferred via electric field between two metal electrodes, which together form a capacitor.

This has had some scattered uses in low-power applications, but it's fairly limited, in part because electric fields, unlike magnetic fields, interact with objects, including such objects as the human body. And electrodes need a pretty high voltage to transmit power, so there are safety issues.

3) Resonant coupling

Here's where things get interesting.

Plain old inductive coupling involves omnidirectional transmission. The field radiates out from the transmitter in all directions; unless the receiver coil is very, very close, it doesn't pick up much energy. That's why cellphones have to sit so precisely on those charging pads.

Then there's resonant inductive coupling, or "magnetic resonance," in which the transmitter and receiver coils are tuned to the same frequency. Thus tuned, they become "strongly coupled" and create an LC circuit, which allows the field to be directed and the receiver to pick up more energy, at greater distances, more efficiently.

(Both inductive and capacitive coupling can be made resonant, but the action is mainly in resonant inductive coupling — though here's a startup using resonant capacitive coupling to charge drones.)

Magnetic resonance has proved itself in lab conditions and prototypes. It was demonstrated by a team at MIT in 2007. The head of that team, Marin Soljačić, went on to found a company called WiTricity. Here's a 2009 TED talk about it:



WiTricity calls its product "mid-range" wireless charging, which means anything from a centimeter to several feet. It makes a development kit meant to enable other companies to create wireless power products. As far as I know, none of those products are on the market yet, but it sounds like lots are on the way, as soon as this year.

Other companies are getting in on the action too. Check out Intel's "power bowl":



There are two competing standards for magnetic resonance too. One — developed by the old Alliance for Wireless Power (A4WP) before it merged into the AirFuel Alliance — is called "Rezence." (Multiple Rezence-certified products, including laptops, are expected within the year.) Then there's the WPC's updated Qi standard (1.2, or "Resonant Qi").

Also, the AirFuel Alliance is allegedly looking into "multi-mode" charges and receivers, which can use both induction and resonant induction.



Magnetic resonance has serious advantages. It works over longer distances than inductive coupling, and at a wider array of orientations, eliminating the need for precise device placement. It can charge multiple devices at once. And, like inductive coupling, it is entirely safe for human beings. (All these technologies have to obey strict federal safety standards.)

But its range is still fairly limited. And it is still less efficient than plugging in — the Resonant Qi standard claims 70 to 80 percent efficiency. That will presumably improve with product development, but still, 20 percent is a lot of electricity to waste. (Efficiency is higher, up to 90 percent, in higher-power applications like EV chargers.)

Of all the WPT technologies, resonant coupling probably has the most potential to shake up the market in the near term.

(A few other resonant coupling companies: Mojo Mobility, Power by Proxi, and Qualcomm's WiPower.)

Far-field power transmission

Lots of people think WPT won't really catch on as a consumer technology until it passes a distance tipping point. As one blunt sales exec puts it in this Fortune story, "If you can’t power four devices simultaneously at fifteen feet, then you don’t have anything."

That brings us to far-field power transmission.

Out past a wavelength, electric and magnetic fields go perpendicular to one another and become electromagnetic waves. The second category of WPT includes technologies that use high frequencies to create powerful electromagnetic waves and then focus those waves, via refraction or reflection, into beams, to transmit energy over longer distances.

1) Microwaves

One form of electromagnetic radiation that can carry energy: microwaves.

A Bellevue, Washington-based company called Ossia has developed a product called Cota, which uses focused microwave beams to charge (multiple) devices at a distance of up to 30 feet.

The base transmitter is a black cylinder about the size of a trash can. Receivers, which are small enough to fit in a AAA battery, send out little omnidirectional pings. When they find a transmitter, they lock on to it — only then does power transfer begin. The receiver and transmitter communicate to route the microwave beams, which can bounce off walls, around objects, including humans.



Ossia had a demo at CES 2016, which apparently was a big hit, but as far as I know there's no product on the market yet. (Ossia is in talks with a number of partners to integrate its technology.)

2) Frickin' lasers

Focused beams of infrared light (i.e., lasers) can also transmit power, which is captured on the receiver side by tiny photovoltaic cells.

Lasers have a lot of advantages, besides being awesome. They can carry almost any amount of power, from milliwatts to several hundred kilowatts, over very long distances, measured in miles. They are being used to recharge drones in flight, and there's talk of using them to beam power down from orbiting solar-power satellites.

They also have drawbacks, principally that they require a direct line of sight between transmitter and receiver. Unlike microwave beams, lasers won't bounce off walls and around corners.

Still, there's work being done to commercialize laser charging for consumers. An Israeli startup called Wi-Charge ("why charge?" get it?) has been developing the technology for years and hopes to license it to hardware manufacturers for products in 2016 or 2017.

The idea is to embed chargers in walls or ceilings, where they will have a line of sight to as much of the room as possible. Thus embedded, the chargers can automatically find compatible devices and begin charging them. It can charge multiple devices at a time, with up to 10 kW of power that's constant over any distance, as long as there's a line of sight. And because it uses retro reflectors (don't ask), it can follow a receiver as it moves around the room, as long as nothing comes between the two.

The beam is not visible. It cuts off immediately if the line of sight is broken (and, Ossia claims, would be harmless even if it didn't). And like many of the wireless power technologies discussed thus far, it can also carry information between transmitter and receiver, allowing the former to determine the status and power needs of the latter.

(See also LaserMotive, a laser charging company focused on non-consumer tech.)

3) Radio frequency

Energy can also be transmitted using radio frequencies (RF) and converted back into power via rectification. Sure enough, there's a company out there working on it.

California-based Energous has a product called WattUp that will charge devices (up to four at a time) using RF (in the 5.7 to 5.8 GHz band) from up to 15 feet away. Hundreds of tiny antennas transmit tiny amounts of power via wifi beams; those beams sync up in a small "pocket" around a cluster of dozens of tiny receivers, which harvest the energy.

There's also a Bluetooth connection between transmitter and receiver that allows the former to precisely locate the latter and assess its power needs. The microbeams can be bounced off walls and otherwise routed around bodies, which can block them.

WattUp will deliver 4 kW to objects within 5 feet (about like plugging in), falling to 2 kW within 10 feet (about like a USB charger) and 1 kW within 15 feet (a "trickle charge"). The Underwriters Laboratory has verified that it works.

The trick for Energous is to find partners and get FCC approval. It doesn't manufacture anything; it licenses the technology, so it has to convince a manufacturer to make the chargers before there are compatible devices (and vice versa). It's a tricky problem. (There are rumors that Apple might partner with Energous — that would certainly raise its profile!)

One big drawback is that the company's efficiency target is 25 percent, which presumably will only hold in ideal conditions. That's a lot of wasted energy.

3) Ultrasonic

Technically this is out of place on this list, since it's not electromagnetic waves but sound waves doing the work, specifically focused beams of ultrasonic waves, which are converted back into electricity via transduction.

There's a bit of drama in this area.

For years, buzz has been building around a company called Ubeam, which promised to use ultrasonics to commercialize easy, fast wireless charging. CEO Meredith Perry, who developed the technology and started the company as an undergraduate in 2011, has been promising "wifi for power" and a "world without wires."

She says ultrasonics are the only way to overcome the weaknesses of other wireless charging technologies. And she convinced enough investors that the company has raised more than $20 million in VC funding.

But recently an engineer previously employed at Ubeam, Paul Reynolds, has posted a highly critical series of pieces alleging that the tech can't possibly live up to the headlines, despite engineers being pressured to say otherwise. He speaks for a growing number of experts casting doubt on whether Ubeam, which has never demonstrated its technology in public, can possibly achieve the range, power, and convenience it promises. He calls it the "next Theranos."

Ubeam investors still defend the technology. We shall see.

The future of wireless charging

This is a long list, and it's not exhaustive. We are likely on the front end of a fierce cycle of hype around WPT. It's a hot area of technology, with a lot of big claims being made and a potentially huge market at stake.

But remember, the only technology that has passed the crucial test of commercial viability is boring old inductive coupling. This market analysis — which says that the WPT market was worth $720 million in 2014 and is expected see a 51.5 percent compound annual growth rate between 2015 and 2020 — predicts that the big action over the next five years is likely to be in magnetic resonance. (Here's another bullish report.)

Inductive coupling, with help from magnetic resonance, will be enough to shake things up in the world of transportation. Or at least that's what I'll argue in my next post.

But it's debatable whether it's enough to have a big impact on the consumer device market. Sitting your phone on a pad (or putting it in a bowl) isn't that much more convenient than plugging it in, at least until the infrastructure is common (and that will take a while).

But if any of the far-field technologies pan out — if wireless power can achieve some range, allowing people to move around and charge multiple devices at once — it could prove revolutionary.

Robust wireless charging at a distance would be incredibly handy for medicine. It could serve in all kinds of military applications. It could be useful in industry, for work in inhospitable environments or work that requires lots of remote sensors.

The really fascinating possibilities open up when wireless charging becomes ubiquitous — when ambient charging is predictably and reliably available in homes, offices, coffee shops, cars, public transit, restaurants, and retail outlets.

I can think of two big consequences beyond the increased convenience.

First: batteries. Americans purchase some 3 billion batteries a year, to say nothing of all the batteries embedded in our phones. That adds up to an enormous solid waste problem. If charging were ubiquitous, devices could shrink or in some cases even eliminate batteries, reducing some of the 180,000 tons of them that go into landfills every year.

Second, think about how much small electronics do for us today. Now imagine if tiny, low-power electronic devices — sensors, monitors, wearable tech — didn't need batteries. They could scale down to almost any size and serve almost any purpose.

The trick, of course, is getting the infrastructure to ubiquity, which is difficult to do before there are devices to take advantage of it … which are difficult to develop before there's infrastructure to support them. It's a huge chicken-and-egg problem (seen in miniature today, with EVs and EV charging stations).

If history is any guide, the transition to wireless charging will be fitful, as chargers, devices, and standards evolve in tandem, technologies improve and get cheaper, and consumers get acclimated.

Amara's Law seems applicable here: We're likely to overestimate the changes wrought by WPT in the short term and underestimate them in the long term. A world with truly ambient, ubiquitous energy transfer is mind-boggling to contemplate. Until then, I'll just be happy if I never have to go outlet hunting in an airport again.