What Is Resonant Charging and How Does It Differ From Inductive? – The Wireless Solution

What Is Resonant Charging and How Does It Differ From Inductive?

Tesla coil transferring energy


You may have heard about wireless charging standards but you may be surprised to hear that there are also two different wireless technologies used in commercial chargers

If you’ve used a wireless charging pad you'll be familiar with inductive charging by now, but you may not have taken advantage of resonant charging which is in many ways a lot more exciting. For instance, it can charge multiple devices over further distances than an inductive charger and it can even charge through objects.

Until very recently both the Airfuel Alliance and the Wireless Power Consortium included both inductive and resonant technologies in their standard specifications, that is until Powermat joined the Wireless Power Consortium in January.

In fact, the WPC has been has been building resonant alongside inductive technology into their chipsets since the Qi 1.2 specification. The most likely reason the WPC has been doing this is because inductive and resonant are not necessarily competing technologies but can be used complimentarily as they both have unique advantages and disadvantages.

 Let’s take a closer look at each of them:


Two Charging Technologies



Diagram of inductive coupling

Inductive charging is the most common wireless charging technology and is likely the technology you use to charge your smartphone.

Inductive charging works by creating an electromagnetic flux between two coils and is most effective when these two coils are closely aligned, rapidly losing energy transfer efficiency the further apart they become (for more info see our blogpost).

The alignment, separation and size of the respective coils determines what is called the “coupling factor” which dictates how much energy is transferred from one coil to another.

While the efficiency of an induction charger is limited to these three factors, it can be hugely effective when they are met, currently reaching charging rates of over 60% efficiency (or a 0.6 coupling factor) in optimal conditions. 




Diagram of Resonant Coupling


Resonant charging or "highly coupled magnetic resonance" was discovered in its modern form in 2007 by researchers from MIT who found that by using a non-radiating flux you could offset some of the drop off in efficiency caused by an increased distance between coils.

If both coils operate at the same resonant frequency it can cause a strong coupling to occur, creating a kind of “energy tunnel” that prevents some leakage over longer distances.

This means that orientation or distance are no longer the main attenuating factors of power transfer but instead it is the bandwidth or “Q factor” (short for Quality Factor), and the efficiency of this factor is being constantly improved over time as research continues. While there's still a significant drop off in efficiency over distance, resonant can travel further distances and removes the need for the coils to be in the same orientation.

On top of this, resonant charging is more suitable for charging multiple devices from the same charger, exciting stuff!

As mentioned however, there is indeed a significant trade off in that the overall efficiency (amount of energy leakage) of resonant chargers can be much lower than their inductive counterparts.

As it stands, this is usually around 30% efficiency at 2cms distance between coils, falling to as low as 15% at 75cm (see this Digi-Key article for more details). Problematic interference can also be caused if there are any other nearby electric appliances operating on a similar bandwidth.



The Implications of Resonant for Wireless Charging


Ultimately, what we have is a trade-off between an efficient system which is constrained by distance and a distant system which is constrained by efficiency.

Although Nicola Tesla had been experimenting with resonant coupling 100 years ago, the discovery of its modern application was seen as a turning point for wireless charging. In fact, the MIT researchers behind it reached the front page of Science, one of the most prestigious scientific journals in the world.

Since then a company called WiTricity, originally backed by these MIT researchers, has been licensing their patent portfolio to developers to help them produce commercial products using magnetic resonance.

The problem is that the technology has never quite flourished in the way that was initially anticipated, at least in the realm of commercial smartphones. But why is that?

Resonant coupling has three notable advantages over inductive charging:

  • More spatial freedom (both in distance and orientation)
  • Multi device capabilities
  • Lower cost of deployment

Despite all these advantages, resonant charging can’t seem to escape the issue that it is just less efficient than inductive charging, and even with all this improved convenience it can’t warrant commercial use due to the drop off in efficiency.

Inductive charging has an end to end efficiency of roughly 60% compared to Resonant’s 40% at optimal positioning, inductive also uses roughly 33% less energy to charge a 27kj battery. Unfortunately, these discrepancies are just too great at the moment for resonant to have a big impact on the market.


Young girl crying


Why You Shouldn't Count Out Resonant Just Yet


However, this doesn't mean resonant charging isn't useful. It’s ability to charge through a variety of materials makes it ideal for public infrastructure deployment.

For instance, public charging companies such as ChargifiAircharge and Chargespot, have been working hard to fill the public arena with charging hotspots that use magnetic resonance to charge through surfaces.

And arguably, even though resonant is falling behind in smartphone wireless charging, it can be more convenient in other areas. For instance, WiTricity have been working with various automotive companies to bring resonant charging to Electric Vehicles (EVs), where this ability to charge over distance may prove highly useful - think charging transmitters in your garage floor or underneath public roads. Supposedly this automotive solution will be able to charge at 90-93% efficiency (due to the larger coils) and provide over 11kW of power.

Witricity automotive solution

There are also huge implications for implanted medical devices such as pacemakers and neurostimulators which can now be charged remotely without the need for close alignment or without risk of heating parts of the body.

WiTricity medical solution

Perhaps with the advent of EV charging and the increased integration of public charging we are not far off from seeing magnetic resonance finally fulfill its original potential.

In fact, the Airfuel Alliance even claim on the basis of a Utah State University study that resonant has the potential to be as efficient as inductive charging, with systems demonstrating 70-80% efficiency in real life use cases.


Pi, the Next Step in Resonant?


A Pi wireless charger next to a Pi enabled phone


Not only does resonant already open up a lot of doors that inductive can’t, another recent development coming out of MIT (once again) could hugely improve the usability of resonant technology.

In 2014 MIT PhD student LiXin discovered a mathematical proof that allowed the shaping of magnetic fields in real time. This so called “shaping magnetic resonance”, if successfully applied, has the potential to transfer power at a similar charging efficiency to inductive charging whilst maintaining all the benefits of a resonant system.

This would mean multi-device charging, at any orientation and without the need for close coupling, all the while providing a substantial level of charge to each device. The way this is achieved is by changing the angle of the magnetic field to perfectly match the angle of the device it's charging. Pretty darn cool if you ask us.

The MIT researchers have since developed a product named “Pi”, a nice echo of Qi which it will no doubt be compatible with if it reaches the market.

Pi won the TechCrunch Disrupt SF in 2017 and as of December 2017 has secured almost $12 million in series A investments. Unfortunately, they've been completely silent on their product in 2018, hopefully this is sign of things in the works. We can only hope.



Final Thoughts


So, if you thought that charging through objects was a myth, or that resonant charging is just a gimmick, we hope to have shown here that neither are true.

Whilst of course we’re big fans of inductive chargers (and we don’t think that’ll change anytime soon), we would be lying if weren’t excited for the future of resonant. In particular, if Pi becomes a commercial success, it could really shake up the way we think about smartphone wireless charging forever.


Two business people jumping for joy


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