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What exactly is wireless charging and how does it work?
Wireless charging technology has been around for more than a century, but its incorporation into gadgets like Apple’s new iPhone range has given it fresh life. Here’s how it works and why it might soon be in everything from homes to robots.
Apple
Wireless charging dates back to the late 1800s, when Nikola Tesla demonstrated magnetic resonant coupling – the capacity to transport electricity over the air by establishing a magnetic field between two circuits, a transmitter, and a receiver. However, it was a technology with few practical applications for about 100 years, except for a few electric toothbrush models.
There are roughly a half-dozen wireless charging technologies in use today, all aiming at eliminating cords from smartphones and laptops to kitchen appliances and automobiles.
Wireless charging is gaining traction in the healthcare, automotive, and manufacturing industries because it promises enhanced mobility and advancements that could allow tiny Internet of Things (IoT) devices to obtain power from a charger many feet away.
The most popular wireless solutions in use today rely on an electromagnetic field created by two copper coils, limiting the distance between a device and a charging station to a bare minimum. Apple has included this sort of charging in the iPhone 8 and iPhone X.
What is wireless charging?
According to David Green, a research manager at IHS Markit, there are three forms of wireless charging. Charging pads use tightly-coupled electromagnetic inductive or non-radiative charging; charging bowls or through-surface type chargers use loosely-coupled or radiative electromagnetic resonant charging, which can transmit a charge a few centimeters; and uncoupled radio frequency (RF) wireless charging, which allows trickling charging over long distances. Tightly coupled inductive charging and loosely coupled resonant charging both works on the same physical principle: a time-varying magnetic field induces a current in a closed loop of wire.
It works like this: A magnetic loop antenna (copper coil) generates an oscillating magnetic field, which causes a current to flow through one or more reception antennas. The quantity of induced current in the receivers increases when the proper capacitance is added so that the loops resonate at the same frequency. This is known as resonant inductive charging or magnetic resonance, and it allows for wider distances between transmitter and receiver while increasing efficiency. Power transfer distance is also affected by coil size. The further a charge can travel, the larger the coil or the more coils there are.
For example, in the case of smartphone wireless charging pads, the copper coils are only a few inches in diameter, drastically restricting the distance over which electricity may be properly transmitted.
However, larger coils allow for more energy to be delivered wirelessly. That’s the strategy WiTricity, a business founded on MIT research a decade ago, has helped pioneer. It licenses loosely linked resonant technology for a variety of applications ranging from vehicles to wind turbines to robotics.
In 2007, MIT physics professor Marin Soljai demonstrated that he could transfer electricity over a two-meter distance; at the time, power transfer was only 40% efficient at that distance, implying that 60% of the power was wasted in translation. Later that year, Soljai founded WiTricity to commercialize the technology and its power efficiency has increased since then.
Large copper coils – over 25 centimeters in diameter for the receivers – in WiTricity’s car charging system enable efficient power transfer at distances of up to 25 centimeters. According to WiTricity CTO Morris Kesler, the utilization of resonance allows for high levels of power transmission (up to 11kW) and high efficiency (more than 92% end-to-end). WiTricity further augments the conducting loop with capacitors, increasing the amount of energy that can be caught and used to charge a battery.
The system is not limited to automobiles: Daihen Corp., a Japanese robotics manufacturer, began deploying a wireless power transfer system for automated guided vehicles (AGVs) last year. AGVs using Daihen’s D-Broad wireless charging system may simply draw up to a charging station to charge.
While charging from a distance has great potential, the public face of wireless charging has so far been charging mats.
“In terms of progress and industry readiness, charging pads have been shipping in volume since 2015; charging bowls/through-surface type are 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 the very short range right now, e.g., a couple of centimeters,” Green explained.
In 2016, just over 200 million wireless charging-enabled gadgets were shipped, with nearly all of them employing some form of inductive (charging pad) design.
After years of lagging behind other cellphone manufacturers, Apple eventually chose a side in September by supporting WPC’s Qi standard, which Samsung and other Android smartphone makers have been utilizing for at least two years.
The first class of wireless chargers for mobile devices appeared about six years ago; they employed strongly linked or inductive charging, which requires users to set their smartphone in an exact position on a pad for it to charge.
“In my opinion, lining it up exactly to charge doesn’t save you a lot of effort over just plugging it in,” said Navigant Research principal analyst Benjamin Freas.
While early adopters and techies embraced inductive charging, others did not, according to Freas.
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