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.

Mingis on Tech: What's up with wireless charging?

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Wireless charging has been around since the late 19th century, when electricity pioneer Nikola Tesla demonstrated 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 

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.

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 a two copper coils, which greatly 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 non-radiative charging; charging bowls or through-surface type chargers that use loosely-coupled or radiative 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

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 induced current 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 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.

In 2007, MIT physics professor Marin Soljačić proved he could transfer electricity at a distance of two meters; at the time, the power transfer was only 40% efficient at that distance, meaning 60% of the power was lost in translation. Soljačić started WiTricity later that year to commercialize the technology, and its power-transfer efficiency has greatly increased since then.