Wireless charging explained: What is it and how does it work?

Wireless charging technology has been around for more than 100 years, but its inclusion in devices such as Apple's new iPhone line has given it new life. Here's how it works, and why it could soon show up in everything from homes to robots.

Wireless charging has been around since the late 19th century, when electricity pioneer Nikola Tesla deomnstrated magnetic resonant coupling – the ability to transmit electricity through the air by creating a magnetic field between two circuits, a transmitter and a receiver.

But for about 100 years it was a technology without many practical applications, except, perhaps, for a few electric toothbrush models.

Today, there are nearly a half dozen wireless charging technologies in use, all aimed at cutting cables to everything from smartphones and laptops to kitchen appliances and cars. 

Wireless charging is making inroads in the healthcare, automotive and manufacturing industries because it offers the promise of increased mobility and advances that could allow tiny internet of things (IoT) devices to get power many feet away from a charger.

cota circuit board Ossia

The wireless charging circuit board used for Ossia's Cota RF technology, which can send power over distances greater than 15 feet.

The most popular wireless technologies now in use rely on an electromagnetic field between two copper coils, which limits the distance between a device and a charging pad. That's the type of charging Apple has incorporated into the iPhone 8 and the iPhone X.

How wireless charging works

Broadly speaking, there are three types of wireless charging, according to David Green, a research manager with IHS Markit. There are charging pads that use tightly-coupled electromagnetic inductive or short range charging; charging bowls or through-surface type chargers that use loosely-coupled electromagnetic resonant charging that can transmit a charge a few centimeters; and uncoupled radio frequency (RF) wireless charging that allows a trickle charging capability at distances of many feet.

Both tightly-coupled inductive and loosely-coupled resonant charging operate on the same principle of physics: a time-varying magnetic field induces a current in a closed loop of wire.

Ikea wireless charging pads Ikea

Ikea's wireless charger line-up, which includes a pad that's capable of charging three devices at once (center).

It works like this: A magnetic loop antenna (copper coil) is used to create an oscillating magnetic field, which can create a current in one or more receiver antennas. If the appropriate capacitance is added so that the loops resonate at the same frequency, the amount of current induced in the receivers increases. This is resonant inductive charging or magnetic resonance; it enables power transmission at greater distances between transmitter and receiver and increases efficiency. Coil size also affects the distance of power transfer. The bigger the coil, or the more coils there are, the greater the distance a charge can travel. 

In the case of smartphone wireless charging pads, for example, the copper coils are only a few inches in diameter, severely limiting the distance over which power can travel efficiently.

But when the coils are larger, more energy can be transferred wirelessly. That's the tactic WiTricity, a Watertown, Mass. company formed from research at MIT a decade ago, has helped pioneer. It licenses loosely-coupled resonant technology for everything from automobiles and wind turbines to robotics.

WiTricity was started by Marin Soljačić, an MIT physics professor who in 2007 proved he could transfer electricity at a distance of two meters; the power transfer was only 40% efficient at that distance, meaning 60% was lost in translation. Soljačić hoped to commercialize the technology.

WiTricity now offers a car charging system with large copper coils – over 25 centimeters in diameter for the receivers – that allow for efficient power transfer over distances of up to 25 centimeters. The system enables high levels of power (up to 11kW) to be transmitted with greater than 92% efficiency end-to-end, according to WiTricity CTO Morris Kesler. WiTricity also adds capacitors to the conducting loop, which boosts the amount of energy that can be captured and used to charge a battery.

The techn ology works for more than cars. Last year, Japan-based robotics manufacturer Daihen Corp. began shipping a wireless power transfer system for automatic guided vehicles (AGVs) based on WiTricity's technology. AGVs equipped with Daihen's D-Broad wireless charging system can simply pull up to a charging area to power up and then go about their warehouse duties.

While charging at a distance has potential, the more common use for wireless charging involves charging pads and mobile devices.

wireless charging iPhone 8 damage battery IHS Markit

"In terms of progress and industry readiness, charging pads have been shipping in volume since 2015; charging bowls/through-surface type are really just launching this year; and charging across a room is probably still at least a year away from commercial high-volume reality – although the new Energous products show this method working over very short range right now, e.g., a couple of centimeters," Green said.

Energous has developed technology that allows wireless charging over a short distance.

Just over 200 million wireless charging-enabled devices shipped in 2016, with almost all of them using some form of inductive (charging pad) type design.

In September, Apple finally chose a side after lagging behind other handset manufacturers for years by embracing WPC's Qi standard, the same technology  Samsung and other Android smartphone makers have been using for at least two years.

The first class of mobile device wireless chargers emerged six or so years ago; they used tightly-coupled or inductive charging, which requires users to place a smartphone in an exact position on a pad for it to charge.

"In my mind, lining it up exactly to charge doesn't save you a lot of effort from just plugging it in," said Benjamin Freas, principal analyst for Navigant Research.

While early adopters and techies bought into inductive charging, others did not, Feas said.

cw boostup wireless charger diagram Belkin/IDG

Belkin's BoosUp wireless charging pad is similar to others in that it contains a copper transmitter charging, a chipset to control the power delivered to a device, and foreign object detection technoogy to ensure objects that shouldn't recieve a charge don't.

Then in September 2012, the Nokia 920 became the first commercially available smartphone to offer built-in wireless charging capabilities based on the Qi specification.

The wireless charging standards battle

For several years, three competing standards groups have focused on setting inductive and resonance charging specifications: The Alliance for Wireless Power (A4WP), the Power Matters Alliance (PMA) and the Wireless Power Consortium (WPC). The latter's 296-member roster includes Apple, Google, Verizon and a veritable who's who of electronics manufacturers.

The WPC created the most popular of the wireless charging standards – Qi (pronounced "chee") – which enables inductive or pad-style charging and short-distance (1.5cm or less) electromagnetic resonant inductive charging. The Qi standard is being used by Apple.

apple watch wireless charging Apple

The Apple Watch, launched in 2015, uses a inductive wireless charging cable, which still requires the device to be tethered to a cord.

The PMA and its Powermat resonant charging specification found success in pilot programs in coffee shops and airports. Starbucks, for example, began offering wireless charging pads in 2014.

Because of the competing standards, support for mobile devices remained fragmented, with most mobile devices needing an adaptive case to enable a wireless charge.

In 2015, the A4WP and the PMA decided to band together to form the AirFuel Alliance; it now has 110 members, including include Dell, Duracell, Samsung and Qualcomm.

powermat charging starbucks PMA/Starbucks

In 2014, Starbucks announced it would roll out wireless charging based on the Powermat specification for its customers in the U.S. at nearly 8,000 coffee shops.

As part of the AirFuel Alliance, Duracell Powermat claims more than 1,500 charging spots in the U.S., and through Powermat's partnership PowerKiss, 1,000 charging spots in European airports, hotels and cafes. AirFuel has also announced wireless charging at some McDonald's restaurants. That, according to Freas, is one way wireless charging could see wider adoption.

AirFuel focuses on electromagnetic resonant and RF

AirFuel has focused on two charging technologies: electromagnetic resonant and radio frequency, which lets users move around a space while their mobile device charges.

"We've seen clear market indicators that resonant and RF are the way to go," said AirFuel spokesperson Sharen Santoski. "Both technologies offer distinct advantages in terms of spatial freedom, ease of use and ease of installation – big factors in creating market value and customer satisfaction. And we believe resonant is the best technology to enable widespread public infrastructure deployment in the near term."

As a result, Santoski said, a growing number of public spots have deployed resonant-based wireless charging stations. "Taiwan is investing heavily, as is China," Santoski said.

AirFuel recently announced a project with the Taoyuan Airport Metro, which is putting Resonant charging in its trains and stations. And furniture maker Order Furniture has created a new line of Resonant-enabled furniture.

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"If in every restaurant and coffee shop you have it, then people will be more likely to use it and get a pad to charge at home," Freas said.

Most of these efforts are still just pilot programs, Freas said, adding that consumers and businesses are less likely to want tightly coupled charging and more likely to opt for loosely coupled resonant charging. That's because loosely coupled charging lets you simply plop down a phone, tablet or laptop on a desktop and have it charge.

WiTricity and wireless charging in vehicles

In July, Dell released a Latitude laptop that incorporates resonant wireless charging from WiTricity. The Dell wireless charger offers up to 30W of charging power, meaning  a Latitude laptop will charge at the same rate as it were plugged into a wall outlet.

dell laptop WiTricity WiTricity

Dell's new Latitude 7285 2-in-1 laptop and wireless charging pad.

But WiTricity's main focus is the auto industry. The company, which is part of the AirFuel Alliance, expects a number of electric car manufacturers to announce wireless charging for their vehicles, according to WiTricity CEO Alex Gruzen.

The company's electromagnetic resonant technology allows power to transfer at distances of up to about nine inches away from a charging pad. That would allow electric cars to charge just by parking on top of a large charging pad.

For example, Mercedes-Benz this year will roll out S550e plug-in hybrid sedans that can use WiTricity's technology; the S550e simply parks over a pad and begins charging even more efficiently than if it were plugged in.

The electric vehicle application is tailor-made for electromagnetic resonant charging, Kesler said. That's because a vehicle doesn't need a charging cable, and the wireless charging pad delivers electricity more efficiently. (Wired charging systems use electronics to convert AC to DC and regulate the flow of power, reducing efficiency to about 86%, Kessler said.)

Qi wireless charging damage your iPhone Wireless Power Constortium

Nearly 50 car models now offer Qi-based wireless charging in their cabins.

"Our wireless charging can be 93% efficient from end to end – from the wall to what's being delivered to the battery," Kesler said.

Wireless charging over distance

This month, Apple surprised some industry watchers by purchasing PowerByProxi, a New Zealand-based company developing loosely coupled resonant charging technology that's based on the Qi specification.

PowerbyProxi was founded in 2007 by entrepreneur Fady Mishriki as a spin-out from the University of Auckland. PowerByProxi has showcased charging boxes and bowls into which multiple devices can be placed and charged at the same time.

The Aukland-based company got its start selling large-scale systems for the  construction, telecommunications, defense and agriculture industries. One such  product is a wireless control system for wind turbines.

PowerByProxi, a member of the WPC's Steering Committee, has also miniaturized its technology and placed it into AA rechargeable batteries, eliminating the need to embed the technology directly into devices. The wireless technology takes up about 10% of the AA battery height.

Apple could use PowerByProxi's technology to expand its use of wireless charging beyond just smartphones – using it, for instance, to charge TV remote controls, computer peripherals or other devices that require batteries.

Though mobile device charging pads are popular, wireless charging technology is also making inroads into everything from warehouse robots to tiny IoT devices that otherwise would need to be wired or powered by replaceable batteries.

Both Ossia and Energous have demonstrated wireless charging beyond 15 feet. Ossia's charger can send about two watts up to several feet, but that drops off quickly as the distance increases. Even at 30 feet, however, the amount of power transmitted is "meaningful," according to Ossia CEO Mario Obeidat, alluding to trickle powering devices so as to maintain their charge.

Ossia Cota Wireless Charging system Mark Hachman

The Ossia Cota wireless power system includes this large cylindrical transmitter as well as receivers. 

"Let's say I'm in an office for eight to 10 hours a day and I'm receiving a half a watt or a watt of power; it's charging my device all the time," Obeidat said. "So if it takes five hours to fully charge that devise, that's fine because you're there all the time."

"I've used both [Ossia and Energous]; the technology works," Rob Rueckert, the managing director at Sorenson Capital, a private equity and venture capital firm, said in an earlier interview with Computerworld.

Rueckert believes charging at distance is a more compelling technology than rival technologies.

how wireless charging works resonant PowerByProxi

How wireless charging works.

Both Energous' WattUp and Ossia's Cota mobile device charging systems work much like a wireless router, sending radio frequency (RF) signals that can be received by enabled wearables and mobile phones. A small RF antenna in the form of PCB board, an ASIC and software make up the wireless power receivers.

Using a multi-antenna management chip about 4x4mm in size, the Cota power transmitter can be built into a variety of form factors – everything from ceiling tiles to  tables, desks, glass, televisions and automobile dashboards.

The transmitter automatically detects Cota-enabled devices and includes a temperature-sensing unit to prevent overheating.

"We call it real wireless power," Obeidat said. "The difference between our technology and others in market, like Qi, is we can deliver meaningful power remotely. Others require you to place your device on the pad. So, effectively you have to give up the device to charge it."

Obeidat also claims Cota charging can work through walls, just like a Wi-Fi router.

"Our technology is agnostic. You can envision having a transmitter in room where it powers a smartphone, a tablet or a smartwatch, all at the same time," Obeidat said.

Ossia has been piloting its technology on electronic labels for products on retail shelves. The labels can inform shoppers of product details or sales without requiring workers to place physical signs or change price stickers.

While some have scoffed at the idea of only transmitting a couple of watts of power over distance, investors have taken the idea seriously. For example, Pleasanton, Calif.-based Energous – an AirFuel member – raised about $25 million when it went public in 2014.

Energous' WattUp charger uses the Bluetooth wireless communication spec. Like Ossia's Cota technology, the amount of wattage WattUp can send is limited. As a result, Energous is focused on powering small mobile devices rather than laptops or batteries that require higher capacities.

energous wireless power Energous

Michael Leabman, founder and CTO of Energous, demonstrates how one of the company's wireless charging routers can send power at midrange and far-range distances.

A single WattUp transmitter can charge up to 24 devices, all under software control that enables or disables charging, according to Energous. The maximum amount of power – 4 watts – can only be delivered to four devices simultaneously. As more "authorized" devices enter a room, the charge to each device drops.

One potential obstacle to adoption of wireless charging at distance is that neither Ossia's nor Energous' can charge Qi-enabled devices; the technology is proprietary.

Only the beginning

Green believes Qi and Powermat provide a great start, but stresses the technology  isn't completely wireless. "Qi has started the conversation about wireless power. There is an important need to educate consumers about what is possible," he said.

By starting with a Qi pad charging, users will come to understand the premise of wireless power and will soon demand a much more flexible, robust solution: power at a distance with the flexibility to use a device while its charging.

"One thing is clear: in 2017, we're not going to see a device offering full-speed wireless charging across a room," Green said. "There's two ends of a scale instead, charging at the same speed as a wire but on a charging pad, or perhaps trickle charging very slowly but at a larger distance away."