Appliance Science: How wireless power works

Need to charge your phone? Just plug it in. But what if you could charge it without wires, transmitting electricity through the air to your phone? It's no pipe dream: on many modern phones, you can charge your devices without wires using a wireless charger.

Samsung offers this feature built into the Galaxy S7 phone: put it on the optional $60 Qi charger and it will charge away without wires. It's not just phones, though -- other small portable electronics use similar systems. Apple uses it on the Apple Watch and may be planning to add this feature to the next version of its iPhone. So, how do these systems work? Let's take a look at the science of wireless power.

There are two main wireless charging standards: Qi (from an industry group called the Wireless Power Consortium) and Airfuel, created by a group called the AirFuel Alliance which includes companies such as PowerMat. Although they differ in the specifics, both work using an approach called resonant inductive coupling. In these systems, the power is transferred between two coils of wire: one in the charging base, and one in the device.

In the base, the power is fed through the charger coil, alternating in direction at a specific frequency. This power flow creates a magnetic field, which alternates as the power ebbs and flows. When the coil in the device to be charged is laid parallel to this (such as when you lay your phone on the charger), this rapidly alternating magnetic field (which alternates at a speed of 100 to 200 time per second) is picked up by the receiving coil in the device. This creates an electric current that charges the device.

If you have read my column on induction cooktops, bits of that might sound oddly familiar. That's because these wireless charging systems and induction cooktops use the same fundamental technology. In an induction cooktop, the magnetic field heats a piece of metal in the base of the pan, while in a wireless power system, the magnetic field creates a current in the receiving coil. In other words, while the induction cooktop uses this technology to create heat, the wireless power system uses it to transfer electrical energy through the space between the two devices. Both systems are using a magnetic field to transfer energy, but with different results.

Constantly sending a current through the coil in the charging base would be a waste of power, so the charger only sends an occasional pulse of energy through the coil. When a device gets put onto the charger, it notices the resulting change in the flow of current and sends a signal to check if the device needs charging. If it responds, then the charger will start to send the energy through the coil. Once the device is fully charged, it sends a signal back to the charger telling it that it is done, and the charger stops the energy flow. This system of signalling means that the charger won't start sending energy through the coil unless it detects a device that requests it.

There is a price to pay for the convenience of wireless charging, though. During the process of creating a magnetic field and inducing the current in the second coil, some of the energy is lost. This loss can be up to 50 percent, meaning that the device being charged only gets half of the energy that flows into the charger. The rest is lost in the process, dissipating in magnetic flux that is not picked up by the receiving coil.

There is also the issue of standards. Qi and Powermat chargers aren't compatible, so if you have a Qi charger but your device uses a Powermat receiver, you are out of luck. That may be changing in future, though, as companies are working to develop chargers that can handle multiple standards. Semtech, for instance, recently showed a prototype that can charge both Qi and Powermat devices by emulating both standards with a single charger. This should, hopefully, mean that the chargers and devices of these rival standards may be compatible in future.

The future also holds new standards, including systems that can send power over larger distances. Apple patented a system in 2012 called near-field magnetic resonance (NFMR) that could send power over longer distances than Qi or Powermat. In the patent, Apple claims that this could send power over a distance of a meter (just over 3 feet) and which could charge multiple devices at once to create what Apple describes as "a wirelessly powered local computing environment." It works by shaping and tuning the magnetic field so that it can charge devices from a longer distance, then detuning it to stem the flow of energy when the device is charged.

So, are we on the cusp of a new golden age of wireless charging? That remains to be seen; you don't have to prove something works to get a patent, so this may be no more than Apple protecting an idea it is experimenting with. But, if it works, it could be a whole new ball game for how you charge your devices.