How Does Wireless Charging Work

by James Hunt

In 1899, the inventor Nikola Tesla began performing the first successful experiments on wireless power transfer. His initial success led him to believe that one day power would be transmitted around the planet without the need for cables. It took over 100 years, but his dream of wire-free power transmission was eventually realized—though perhaps not by the methods he envisioned.

When creating phones and tablets, manufacturers are faced with the challenge of giving the device a long battery life, keeping it lightweight, and making charging as painless as possible. Wireless power, which makes recharging your phone as easy as putting it down, could be the solution to that last part. But how does it actually work? And, perhaps more importantly, how safe is it?

Modern wireless power draws on the same principle that Tesla investigated over a century ago: induction. Electromagnetic induction—using an electromagnetic field to transfer power between two objects—forms the basis of all modern wireless charging, as well as things like contactless payment, cooktops, and wireless speakers.

In a practical sense, the way induction works is simple: First, you feed power to a base unit or charging station that contains a “transmitter” coil. An electromagnetic field forms around the transmitter and when a second “receiver” coil comes near enough, the receiver coil interacts with the magnetic field to create an electric current. By putting the second coil inside another device, you can wirelessly transfer power from the base to the device.

Most induction chargers only operate over a short distance, however, and while physical contact between a device and its base unit isn't necessary for induction to work, the fields generated lose so much power as the devices get farther away that it's usually the only way to get the two coils close enough.

As for safety, there's really nothing to worry about. The average induction charger creates a field no more dangerous than radio waves, and it isn't strong enough to have any effect on the human body. If anything, plugging in and unplugging a cable is more dangerous because there's a minute chance it could fray and shock you. By contrast, induction hardware can be safely encased in thick plastic and still work. This is why electric toothbrushes have long used induction to charge: The units can remain sealed and waterproof.

Sounds great, right? So why don't we use wireless charging all the time? For starters, it’s slow going. While wireless charging has improved dramatically over the past few years, wired charging is still generally faster. Also, the process creates a lot of waste heat, so much so that some Samsung charging pads have fans to keep everything cool.

The big issue is practicality, though. You can easily use your phone while it's plugged into a charger, but it's tricky to hold your phone up to your ear while it's resting on a wireless base station.

But things are changing.

Returning to Tesla's original experiments, an effect called Resonant Inductive Coupling allowed the inventor to safely transmit power over several meters. Perhaps the most popular wireless charging standard, Qi, has recently been updated to allow a version of this to be implemented in compatible devices. The result is that the charging range has increased to four centimeters.

It might not sound like much, but it's a start. In the future, wall-sized charge stations might be able to transmit power to multiple devices in multiple rooms as you move about your house. It may have taken over a century to get to this point, but we're closer than ever to wireless power transfer becoming commonplace. It's what Tesla would've wanted.