No wires, no tangles

Recent developments in wireless charging technology

By Gabriela Ehrlich

Wireless charging is expected to prove highly convenient for consumers, yet the market is being slow to develop. One of the reasons could be that a standards war – albeit a small one – takes place behind the scenes. At CES e-tech spoke with representatives of the two dominant consortia in this area and explored the latest developments as well as the most promising perspectives for the future.  

From infancy…

In 2010, when e-tech first participated at CES, wireless charging was in its infancy and was generating a widespread buzz. Powermat was the first company to promote a usable charging product designed for mobile phones. The lure of being able to drop a phone onto a surface without having to plug it in seemed pleasingly futuristic. Of course the dampener was that only phones equipped with a special case could be charged in this way. Also, the phone couldn’t just be dropped any old way, it had to be positioned precisely on the small charging spot on the mat.

…to adolescence

Since then the technology has advanced rapidly and many companies have joined the fray as part of one of the bigger consortia addressing this space.

The reason for this growing interest in wireless power transfer (WPT) is linked directly to the adoption of modern consumer electronics whose need for power is growing faster than their ability to store that power. As a result these devices require frequent charging. Ultimately, WPT will allow users to recharge any portable device reliably anywhere, anytime without the constant burden of carrying and deploying different charging accessories for each of them.

Freedom requires interoperability

Today, most approaches to WPT technology are largely non-interoperable. This is particularly true for multimedia systems and devices. A vision of freedom from multiple chargers depends on WPT integration in the physical environment while maximum interoperability demands the alignment of standards.

The common prerequisite for all devices is that WPT should neither disrupt normal performance nor affect negatively the functionality of its infrastructure; i.e. the consumer will want to be able to use a table or desk top in the normal way, irrespective of its charging capabilities.

Reducing e-waste

Another big advantage WPT holds out is the reduction of e-waste from batteries and a decrease in the number of external power supplies needed to charge miscellaneous devices. In a WPT world many devices will operate using the same charger and cable. This approach will also result in better use of primary resources.

Not that new

The commercial application of WPT has its origins in the work of Nikola Tesla in the early 1900s. It is now well-established in several industrial and specialized application areas; for instance supplying power to "people mover" systems in airports, materials handling systems in manufacturing and warehousing and "mission critical" control systems that isolate the power supply from environmental disruption.

Who is going to win?

When talking to Dr Kamil Grajski, the Technical Area Manager of IEC Technical Committee (TC) 100 TA 15: Wireless power transfer, at CES, e-tech wanted to find out more about the technologies involved and understand better if and how this standards war can be resolved.

It turns out that the technologies in themselves are too different to allow for alignment. However, there is light at the end of the tunnel. All of the major companies that participate in the biggest consortia have hedged their bets, preparing for whichever technology will ultimately be adopted. Now that two of the three biggest consortia have merged, it looks as if the need to grow the market has finally triumphed.

Wireless charging in the IEC

In the IEC work on WPT is split into two distinct TCs because of the wide variation in the power demands of various devices and systems. An electric vehicle requires in the range of 2 000 to 5 000 watts or more to charge its battery, while the latest smartphones, tablets and laptops need between 10 and 50 watts. The use cases and power and regulatory requirements are all quite different at the two ends of the charging spectrum.

TC 100/TA (Technical Area) 15: Wireless power transfer, is in charge of developing international publications for multimedia systems and equipment; TC 69: Electric road vehicles and electric industrial trucks, prepares the International Standards needed for the wireless charging of electric vehicles but also scooters and buses, among other things.

Major players

Several industry-led consortia are developing industry specifications in the WPT space. They range from those focused on specific WPT technology approaches to those seeking to represent the needs and technical requirements of one or more vertical market segments.

And while miscellaneous interest groups may have WPT as part of their agenda, three consortia (two of them now merged – see below) focus fully on this approach:

The Wireless Power Consortium (WPC) specification uses electromagnetic induction of the type commonly referred to as "tightly-coupled" at an operating frequency of 105 to 205 kHz with a typical power output of around 5 to 7,5 W and with an expected maximum power output that is targeted to reach 15 W. The technology is promoted under the Qi brand and has been adopted by a range of consumer electronics and power product manufacturers. The consortium now has a membership of just over 200 companies and by its own unaudited declaration its members have sold over 50 million products since its inception in 2010. While that may sound a lot, it should be put into perspective. The consumer electronics market alone ships between 2 and 3 billion products every year. One of the major chip manufacturers sells more than 70 million chips each month.

Products include anything from mobile phones (usually charged in cradles) to electric tooth-brushes or water heaters that commonly stand on a platform with power transmitted by a vertical connector. To achieve power transfer, the Qi technology requires relatively precise alignment and proximity between charger and device. This is achieved through accessories such as charging "sleeves", pads or battery covers.

The technology has some drawbacks in terms of heat development and the quantity of power that can be transferred to a device. Also a metal object on the charging surface will experience a temperature rise thereby possibly disrupting the charging cycle. The WPC has a Kitchen Appliances Working Group that is working to extend the inductive technology to higher power levels. In addition, the WPC has announced its intention to provide a resonant solution that is backwards-compatible with the Qi installed base.

The Alliance for Wireless Power (A4WP) focuses on non-radiative near-field magnetic resonant coupling or so called loosely-coupled WPT in the 6,78 MHz band. The maximum power output for the highest rated class of chargers is between 50 and 70 W. The technology is branded Rezence. This industry-led consortium was established in 2012 and brings together around 150 companies.

The Rezence technology allows for almost any surface to be turned into a wireless charging "port". Car dashboards, kitchen tables, office furniture, restaurant counters and retail shelves are all possible candidates. Power can be transferred simultaneously to several devices, including laptops, and these devices can be positioned freely anywhere within the charging surface without the need for precise alignment. For the main part, when small metal objects such as coins and paper clips  are placed on the charging surface, they will not interfere with charging. 

The Power Matters Alliance (PMA) promotes an induction-based power transfer technology that is similar to the WPC technology with slight variations in terms of frequency and protocol. It is branded as P and has around 70 members. What the PMA has pioneered is a vision of an interconnected network of wireless charging stations deployed within venues that may be public (e.g., airports) or private (e.g., coffee shops) and that deliver not only wireless power, but also serve as a gateway to provide additional media services. 

At CES the two consortia A4WP and PMA announced their merger, effective 30 June 2015. The resultant new organization aims to accelerate market development for WPT in the consumer electronics world and will cover a broad range of devices from wearables to smartphones, tablets, notebooks, laptops and more. The two organizations explained that having different standards out there has been confusing for the market. They aim to emulate the example of smartphones which today support a multiplicity of radio technologies, including NFC, Bluetooth, Wi-Fi, 3G, 4G, and so on. Each is backed by a strong consortium and ecosystem. In their view the same should be possible for devices in the WPT space, with the market deciding on the best technology to use in each case.

Technology overview

WPT comprises a selection of different technologies based on electromagnetic induction, magnetic resonance, electric field coupling, microwave radio transmission and microwave energy harvesting (detailed descriptions can be found in IEC TR 62869:2013). Among the differentiating features are available power levels (mW to kW) and the precision required in terms of the physical alignment between the power source and the device to be charged (mm to m scales).

Electromagnetic induction

Tightly-coupled WPT
An alternating electric current flowing through a coil (source) generates a magnetic field that acts on a receiver coil (sink) to produce a current within it. Electric power is transferred between the source and sink. Both must be in close proximity to achieve a magnetic coupling between the two coils so as to realize high transfer efficiency. The so-called coupling factor is generally ~1 (very tightly-coupled). If this degree of precision is not met, there is no transfer of power.

Magnetic resonance

Loosely-coupled WPT
Magnetic resonance is a special case of electromagnetic induction. Here the source is a resonant coil and series capacitor as resonator, with a corresponding sink element consisting also of a coil and series capacitor as a tuned resonator. Electric power is transferred through the electromagnetic resonance between the source and sink. By matching the resonance frequency between both it is possible to transfer electric power over a relatively long distance (mm to m). Here the coupling factor can be much lower than <<1 (loosely-coupled). That means that even at a distance or with partial overlap, transfer of power is maintained.

Conductive charging

Another WPT technology that is relatively rarely discussed is conductive charging. Conductive charging requires a physical connection between the electronic device’s battery and the power supply. The need for a metal-to-metal connection between the charger and the device requiring charging is one of the main drawbacks of this method. To accomplish charging without the use of physical cords connected to wall outlets, special attachments are fitted with technology that can detect when the device makes connection with the power source, often a charging base. These are designed to distinguish between metal and other surfaces to avoid the risk of electrocution.

Several small companies at CES promoted products, mostly for the charging of mobile phones.e-techinterviewed a small company from San José, US, which received the 2015 CES Award for its innovative approach for a WPT transparent table top. The technology uses a tiny proprietary microchip. The company claims power transfer efficiencies of 95% at very high speeds, without the generation of heat or electromagnetic field radiation, and a charging efficiency as high as that achieved when plugging the device directly into a charger.

The efficiency argument

Several of the interviewees took higher efficiency as a key argument in promoting their approach, but efficiency doesn’t necessarily translate into faster charging. High efficiency simply demonstrates that little energy is lost through heat during the charging of the device. How fast a device is charged depends on how much energy can be delivered to the device.

Magnetic resonance technology delivers 6,78 MHz and between 50 and 70 W – the levels needed to charge a laptop and a smart phone simultaneously – compared with electromagnetic induction, which is limited to between 105 and 205 kHz and 5 to 7,5 W.

What’s next?

To circle back to the original premise of this article: is a standards war blocking adoption of WPT?  Perhaps the answer is that there never was a real standards war.  The market has absorbed the capabilities of the technologies available and is now starting to decide which one is best for a given application. 

With broader adoption of WPT the ecosystem will grow both in breadth and in size with a vision of ultimately building a widespread wireless charging infrastructure.

Going forward, WPT is under consideration for example for medical devices in hospital or home settings. Such devices can remain completely sealed, without dangling wires, and become much more movable and portable. High consumption devices such as refrigerators, washing machines or office machines may also benefit from WPT in the near future.

Other uses of WPT will include integrated circuits in complex miniature machines, human implantable medical and prosthetic devices or microwave beam WPT from orbiting power satellites.

Electromagnetic induction Electromagnetic induction (IEC 1664/13, fig. 6 on p. 25 of IEC TR 62869)
magnetic resonance Magnetic resonance (IEC 1665/14, fig. 7 on p. 25 of IEC TR 62869)