How Does Wireless Charging Work? A Scientific Explanation

All electronic devices require a power source. Most objects can just be plugged into a wall socket, and the gadget will never run out of juice – blackouts notwithstanding. Portable devices, however, require portable sources of electricity, usually in the form of batteries. While many lightweight devices accept disposable batteries, a growing number rely on built-in rechargeable packs that can be periodically topped off.

Initially, built-in batteries were primarily recharged by plugging them into wall sockets via cables. This technique is still commonplace, but wireless charging is quickly becoming more popular. Unlike wired charging, you don't have to plug your battery-powered device into anything, but this raises more questions than it answers. 

For instance, if you're not plugging your device into a wall socket or power outlet, how is the electricity flowing into the battery? Didn't all our science teachers tell us in school that you need a physical connection to create a circuit? Well, yes, but wireless charging goes beyond high school science lessons. In order to understand how this technology works, you need an electrical engineering degree from an accredited college. Of course, if you don't want to spend four years and over $30,000 pursuing one, you can always read this article.

If you know how a radio works, you know how a wireless charger works. Well, that's actually an oversimplification, but not by much.

To put it as simply as possible, a wireless charger takes an alternating electrical current, uses a frequency converter to change the energy into a medium that doesn't require wires, and then beams that medium out through a coupling element. A receiver, which usually mirrors the coupling element, then accepts the electromagnetic field that has been created before pushing it through a frequency converter to change it back into an alternating current, and finally shoots that electricity into the receiving device. 

The exact process changes depending on the mediums in play, as some are better in certain circumstances than others, but at the end of the day, most wireless chargers function the same as wireless communication devices but with different frequencies to accommodate energy instead of data.

Arguably the most common and safest form of wireless charging mediums is magnetic waves, but it is one of many. Wireless chargers can also use electric fields (which are technically different from electric currents), radio waves, microwaves, and even different spectrums of light. However, there is no such thing as a universal coupling element, so each medium requires a different type. These include coils of wire, metal plates, antennae, magnets, and even lasers. Given the sheer number of medium choices, you might wonder how anyone chooses what kind of wireless charger they want. Primarily, it all comes down to distance.

When most people talk about wireless charging, they think of pads you can use to charge your smartphone and those tiny stands you can place electric toothbrushes on. This is not surprising since those are the most common styles of wireless chargers and all use the same method — inductive coupling.

Inductive coupling converts alternating electrical currents to magnetic fields and sends them out using copper wire induction coils. The receiving device then uses its own induction coils to accept the magnetic fields, which are then converted back into electric currents via the same process in reverse. Essentially, these coils function like a pair of transformers, and because an inductive coupling field's frequency and antennae are so small, power drops off exponentially the further away the receiver is from the transmitter. This issue can be fixed by using more power or increasing the size of the transmitter and receiver coils, but that isn't feasible with devices like cell phones, which is why you have to place them on top of the wireless charger.

Inductive coupling, especially for phones, currently comes in two standards, Qi and PMA. Qi chargers use 100 to 205 kHz fields, whereas PMA uses 277 to 357 kHz fields. Furthermore, since Qi and PMA devices utilize different connection protocols, Qi devices aren't compatible with PMA chargers and vice versa. Regardless, Qi wireless charging is widely considered the standard for most inductive coupling wireless chargers. 

Work is underway to utilize inductive coupling for electric vehicles. One day we could see gasless cars charged in their own garages or while driving down roads. However, progress is slow due to prohibitive costs, potential damage to roadways, and possible threats to people with pacemakers.

Since inductive coupling uses magnetic fields, it is relatively safe for use around human bodies. Moreover, because inductive coupling is the most common form of wireless charging, one might assume that most, if not all, wireless chargers are human-friendly. However, this couldn't be further from the truth.

Capacitive coupling, also known as electrostatic induction, creates an electric field between two electrodes or conductive plates. These wireless chargers still convert alternating electrical currents to fields in much the same way as inductive coupling chargers, but since they rely on electrical fields instead of magnetic ones, capacitive couplers are less vulnerable to interference and don't need as much shielding. This cuts down on a wireless charger's mass and manufacturing costs. Moreover, capacitive coupling chargers don't have as strict alignment requirements, so devices can be a little more askew on capacitive couplers and still receive power.

In every field of science, advantages usually come with a few caveats, and capacitive coupling is no different. Unlike inductive coupling's magnetic fields, capacitive coupling's electric fields don't cooperate with the human body. At best, flesh and blood can impede the flow of energy. At worst, capacitive coupling can injure anyone who gets in the way. Moreover, capacitive coupling's performance is limited to low voltages, as high voltages can be dangerous to the electrodes and also produce ozone.

[Featured image by BaronAlaric via Wikimedia Commons | Cropped and scaled | CC BY-SA 3.0]

So if inductive coupling chargers produce magnetic fields — but they aren't magnets — while magnetodynamic coupling chargers utilize magnets, what happens when you combine a magnetic field-producing device with a magnet? The answer is yet another breed of wireless charger.

Electrodynamic wireless power transfer is half inductive coupling and half magnetodynamic coupling. Everything starts at the transmitter, which is a coil that produces a magnetic frequency. The receiver also utilizes a coil, as well as a permanent magnet attached to a rotor. The magnetic field interacts with the receiver coil's magnet, exerting torque on it. This mechanical motion produces voltage and power to the attached device. So what sets this charging method apart from others? For starters, most wireless chargers that produce magnetic fields create high-frequency waves, whereas electrodynamic wireless power transfer devices utilize low-frequency fields in the 10s to 100s Hz range.

Electrodynamic wireless power transfer's magnetic fields pass through electrically conductive media, and their frequencies are much safer for humans, making them ideal for charging biomedical implants. Electrodynamic charging can potentially be used in rooms full of people to charge their implants, as well as power cell phones and other devices, without causing any negative side effects. 

However, while distance hasn't posed much of an obstacle to the amount of power a device receives, during experiments in 2013 and 2014, the electrodynamic wireless power transfer receiver sizes were too large for the researchers' tastes. As such, it might take a while until this charging system becomes commercially available.

When most people hear the word microwaves, they think of microwave appliances. These devices use microwaves (electromagnetic radiation in the centimeter to millimeter range) to produce thermal energy. However, microwaves — the energy, not the appliance — can do more than just heat up food.

Researchers are hard at work figuring out how to channel microwaves to provide power. In 2013, the Physicist Hatem Zeine unveiled the Cota wireless power transmission system, manufactured by Ossia, which used microwaves to beam energy to a receiver 30 feet away. His device transmitted power using the same microwave frequency as Wi-Fi routers, which made it safe for humans.

Other researchers have also gone one step further and tried to develop devices that generate energy while they beam it. One such solution was to use a solar power satellite that absorbed sunlight, converted it into energy, and then transformed the energy into microwaves that would be beamed down to a collection facility. Sounds promising, but depending on the wavelength, the proposed satellite's antenna requirements would have been astronomical. Beaming the power in shorter wavelengths could cut down on antenna size, but the microwaves would be more susceptible to atmospheric absorption. There's always a catch.

Most people know that matter comes in three states: solid, liquid, and gas. However, there is a fourth state, plasma, that only exists under certain conditions. This highly specialized matter has several amazing properties, including the ability to carry tons of electricity in the blink of an eye — or should we say in a flash of lightning?

Atmospheric plasma channel coupling, also known as an electrolaser, is yet another form of wireless charging that utilizes a laser, but instead of using the laser as a channel to provide energy, it uses a laser to create the channel. This laser superheats surrounding gasses until it ionizes them and creates a plasma channel. This laser-induced plasma exists for a brief second but is superconductive enough to form a powerful electric arc. The result is something akin to a lightning bolt, mostly because lightning is actually plasma.

Electrolasers have numerous functions, including a long-range taser that can incapacitate or even kill its target and a method to guide lightning. While the latter can keep lightning bolts away from potentially sensitive targets, scientists in France and Germany teamed up to create the Teramobile, a laser-induced plasma channel device that could agitate clouds with lasers and harvest the resulting lightning bolts. 

While that is one form of atmospheric plasma channel coupling, another form could come in the shape of a patented device that would presumably create a plasma channel and then shoot energy along it to a charging rod. Using electrolasers as a wireless charger is still in its infancy, but one day we might be able to farm and deliver gigajoules worth of energy at the speed of lightning.