Monday, 23 October 2017
News Tech

How Qi wireless charging works, and why it hasn’t taken over yet


Enlarge / The Qi-compatible Nexus 5 on the Nexus Wireless Charger. New chargers will be able to increase the space between the device and the pad.

The Qi wireless charging standard from the Wireless Power Consortium (WPC) is having a relatively good year. Now is a good time for a bird’s-eye view of the technology—how it works, what it’s for, and what its prospects are. This is also a good time because millions of Apple ecosystem users are about to get their first sampling of Qi when the iPhone 8 and iPhone 8 Plus launch this Friday.

Qi has appeared in phones of various stripes for more than five years, and many people are already using it. The basic tech has been used for consumer products like razors and toothbrushes for a while, plus a variety of non-consumer tools.

Even if you haven’t used Qi, you may have seen Qi wireless charge pads at airports. In 2014, Verizon installed Qi wireless charging stations in several US terminals, from JFK to LAX. You’ll find them in plenty of other places, too, including devices like the Samsung Galaxy S8 and the US version of the LG G6.

The adoption story is not all rosy, though. Google’s Nexus 4 phone supported Qi back in 2012, but the more recent Pixel phone doesn’t. A couple of years ago, Starbucks stores went with a completely different standard, although interoperability may be possible.

If wireless charging has been around for years, why has it struggled to become normal? We’ll get into that shortly.

But first, for those who are just getting introduced to Qi, we’ll go into the basics of how it works.

Inductive power transfer

The primary tech behind Qi is called “inductive power transfer.” Let’s be clear: a wire is still involved. There has to be a charging base station, and that needs to be connected to a power outlet. You just don’t have to plug your mobile device in. Instead, you place it on the base station.

The base station doesn’t activate unless a compatible device has been placed on it. The station determines this by sending an intermittent test signal to check if a Qi-compliant device is present. The mobile device responds to this ping by communicating the received signal strength.

At this point, the wireless charging process begins. The base station and the mobile device each have coils—a transmitter coil and a receiver coil, respectively. The transmitter’s coil generates an electromagnetic field that induces a current in the receiver’s coil.

The receiver sends an error signal to the transmitter that simply shares a value equal to the difference between the required power level and the actual power level. The transmitter adapts its output to achieve zero difference between the requested and delivered levels. All this happens at a communication speed of 2 Kbit/s or less. When the receiver’s charge is complete, it essentially tells the transmitter it can go to standby mode.

The first announced wireless charging pads for the iPhone 8 from Apple’s partners do 7.5 watts. The Qi standard allows up to 15 watts, which some Samsung chargers already support. But while these theoretical numbers seem comparable to or even higher than wired chargers (the charger that ships with iPhones is only five watts), these numbers aren’t everything. The result is a slow, steady charge for the mobile device, but actual user experiences show that wireless charging is generally much slower than wired charging.

It’s worth noting that placement is key. The coils are usually intended to be only a few millimeters apart, and a productive coupling between the coils requires accurate positional alignment. But how does the user know where to place their phone on a charging pad?

This is typically addressed in one of three ways. The charging pad or base station can have visual or tactical signifiers of the optimal position for the phone; this is cheap and easy, but it presents challenges when dealing with phones of different sizes and configurations. Alternatively, a charging station might have a coil that moves to align with the coil in the device, allowing you to place it wherever you want, or for a similar result, an array of coils, where specific coils are activated in proximity to the device’s placement.

Inductive charging has been at the heart of Qi in the past, but more recently, the Qi standard now also supports a similar technology called “resonant wireless charging.” Resonant charging is based on some of the same principles, but the use case is different. It can charge over longer distances, but at a lower transfer efficiency. Many newer devices support both modes, but details on the new iPhones are still scant.

Adoption and alternatives

So if Qi has been around for years, why do most people still use wires?

As is so often the case in tech, competing standards are a factor. In addition to Qi and the WPC, we have the AirFuel Alliance. It was born of a merger between the Power Matters Alliance (PMA) and the Alliance for Wireless Power (A4WP) in 2015, but its market saturation lags behind that of Qi.

In case things weren’t confusing enough, the two standards under the AirFuel Alliance are not interoperable. But prospective AirFuel-compliant devices would ship with both technologies in the same product, offering some flexibility in how and where you charge.

In any case, competing standards fracture the market and make adoption less attractive for consumers and manufacturers alike. Even if every standard is supported by one mobile device, eliminating the frustration, that added cost is inevitably passed to consumers.

There’s also the previously noted fact that, in practice, Qi is slower than wired charging. And there’s Apple’s lack of support—until now, anyway. Yes, there are more Android handsets out there than iPhones. But any addition of a major platform can only help the Qi standard. Now that the iPhone 8, iPhone 8 Plus, and iPhone X support Qi, we can anticipate an acceleration of adoption.

Just don’t expect Qi to replace the wire in the immediate future. As you’ve seen, refinements of both convenience and performance must still be made, and there’s always the threat of a newer and better technology around the corner.



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