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

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.

Credit: Apple

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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