How Qi wireless charging works?
2021-06-12
Qi has been an Android staple for a while, and now it’s coming to iPhones, too.
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.
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.
But first, for those who are just getting introduced to Qi, we’ll go into the basics of how it works.
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.
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.
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.
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.
But first, for those who are just getting introduced to Qi, we’ll go into the basics of how it works.
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.
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.