March 22, 2020

The Challenges of Designing a Free-Position, Multi-Device System

The problem with nearly every modern implementation of the Qi Standard is that these chargers have next to no flexibility when it comes to positioning.
The Challenges of Designing a Free-Position, Multi-Device System
Jake Slatnick
The Challenges of Designing a Free-Position, Multi-Device System

The most widely adopted wireless charging protocol is the Qi Standard (pronounced “chee”). Qi-based wireless power can be found in cars, smartphones, and many other devices and applications from leading companies in every industry.

The problem with nearly every modern implementation of the Qi Standard is that these chargers have next to no flexibility when it comes to positioning. Devices need to be perfectly aligned with the center of the charger in order to receive power. This need for precision is a fundamental flaw in the user experience: wireless charging is meant to be convenient above all else, but the need for perfect alignment does not fulfill that promise.

How Qi Works

The Qi Standard transfers energy between the transmitting charger (Tx Device) and the receiving device (Rx Device) using a magnetic field. More specifically, there is a coil that transmits power and a coil that receives power.

You’ll notice that each coil has a hole in the center. This hole typically indicates the coil’s active area, commonly referred to as the “hot spot” or “sweet spot.” In a standard Qi system, the transmitting and receiving sweet spots need to be aligned in order to transmit power, which strictly limits how you can position a device. When the transmitting coil and receiving coil are aligned, a magnetic field is formed, and energy can flow freely between them.

The state of the industry today is such that all the components that make up a wireless charger are available off-the-shelf. The transmitting coils and the chipset that powers them can be purchased from suppliers all over the world. However, all of these parts from all of these suppliers are functionally equivalent, and are designed for limited position technology.

This presents a large engineering barrier if a manufacturer wants to innovate beyond the current format. The process of developing core wireless charging technology for end product companies can be long, expensive, and impractical. For that reason, nearly every modern wireless chargers has limited positioning – it’s essentially the only available chipset functionality that can be purchased off-the-shelf.

In order to solve the limited-position user experience problem, there needs to be a transmitting coil design that can deliver full surface coverage, and a chipset that can power it. Ultimately, a revamped solution would require exploring an entirely unique engineering approach and building a system from the ground up.

Challenges of Free-Position, Multi-Device Charging

While it might sound like a modern-age feat, the concept of transmitting power through a magnetic field has existed for more than a century. Nikola Tesla, the famous inventor, filed a patent in 1897 for the Tesla Coil; his design is the basis for wireless power transfer as we know it today.

Starting with Tesla’s discovery of wireless power transfer through magnetics, companies have been experimenting with transmitting power between two perfectly aligned coils since the last few decades of the 20th century. The Qi Standard, which was established in 2007, continued to build off that model and has been standardizing the single-coil to single-coil implementation ever since.

Despite how long the base technology has been around, transmitting systems that can charge multiple devices using free-position technology present a whole new set of challenges that are not currently well-understood. These challenges stem from misalignment, magnetic field interference, the need to navigate protocols that weren’t designed with free-position in mind, and other limitations related to physics.

In order to achieve free-position charging, and therefore fix the fundamental user experience flaw of limited-position technology, the following challenges need to be addressed:

1. Remaining Within the Confines of Qi

The first major hurdle of free-position technology is the requirement to stay within the confines of the Qi Standard.

There are many different ways to facilitate wireless charging, and some past attempts have even leaned in the free-position direction. As an example, Qualcomm engineered a standard called A4WP (now known as Rezence). However, Rezence – and all other standards at the time – was not adopted on a global scale.

The Qi Standard, on the other hand, was adopted by Samsung in 2013 and by Apple in 2017. Qi has since cemented itself as the global standard across verticals including consumer electronics, automotive, and many others. Once a standard is globally adopted and billions of dollars are collectively invested, it becomes increasingly difficult for other standards – both new and existing – to compete. Since Qi is the one that was adopted by the masses, it’s necessary to operate within its boundaries.

2. Engineering a Multi-Coil Matrix

The fundamental method of transmitting wireless power (as described in the “How Qi Works” section above) involves aligning two coils to create a magnetic field between them. In order to keep the process efficient and Qi-compatible, the transmitting coil needs to be about the same size as the receiving coil (or slightly smaller than the width of an iPhone).

Given this constraint, in order to increase freedom of placement, you can’t simply make a larger coil with a larger hot spot. Instead, it’s necessary to construct a multi-layer array of coils that are nested together so that the sweet spots nearly overlap, providing full surface coverage.

Ideally, the solution for a free-position coil matrix has three physical characteristics: it should be thin, highly manufacturable, and precise. With existing coil technologies, meeting those three requirements isn’t easy, which is why new directions must be explored.

3. Finding the Receiving Sweet Spot Quickly

When a device is placed on the surface of a charger, the charger needs to recognize it’s there. It can’t just spew power all day, waiting for a device to arrive and soak it up. When it does recognize that a device is present, a magnetic field must form to facilitate the flow of energy. Additionally, the newly- introduced magnetic field needs to be created in a way that avoids misfires related to non-compatible objects.

The Qi protocol can detect and transmit power to a device in a way that is safe and reliable. However, because it was designed with limited-position charging in mind, it’s slow and power intensive. In a single 24-hour period, simply by searching for a chargeable device, a single coil transmitter can emit between 3,000mAh and 5,000mAh worth of energy, and can take up to one second to find a device. When scaling that up to a 16-coil or 18-coil system, the daily power output becomes substantial, and the timing to find a device can take several seconds. That may not sound like a lot, but in a world where user experience is king, that’s far too long.

A free-position system requires the ability to rapidly locate a chargeable device, determine the precise location of its sweet spot, and start delivering power immediately. The speed at which this process unfolds greatly impacts the user experience – time is of the essence, so there’s not a millisecond to spare. The power usage specs impact the charger’s eco-friendliness and its ability to pass state and/or federal energy standards, so it should be low-power and energy efficient.

4. Maximizing Efficiency and Managing Heat

Power needs to be transmitted to its coil. The most logical next step is to activate the nearest coil in the matrix. However, based on laws of physics, if a transmitting and receiving coil are offset, the resulting misalignment will make the process less efficient. Low efficiency translates to significant power loss, and power loss creates heat.

Similar to many other electronics and mechanical systems, managing/decreasing heat is the best way to increase overall performance. In the case of wireless charging, too much heat will stop a device from charging altogether. The best way to manage heat is to eliminate power loss by maximizing your efficiency.

In a free-position system, chargeable devices are nearly always offset with surrounding coils. To create a system that’s as efficient as possible, the magnetic field needs to be manipulated so that it’s positioned directly under the receiving sweet spot.

Wireless charging specs frequently tout their maximum efficiency outputs. However, that efficiency can only be achieved when a device is perfectly aligned. When looking at a free-position system, considering the inevitable misalignment, the most important factor is its minimum efficiency. The closer the worst-case efficiencies are to the best-case efficiencies, the better the overall charging experience will be.

5. Detecting Metal and Foreign Objects

As a matter of safety, a wireless charger needs to know that the energy being transmitted is ultimately finding its way to the right place. If it’s not reaching the receiving device’s coil and making it to the battery, that means it’s going somewhere else. If that somewhere else is another object, the lost energy can cause damage and produce dangerous amounts of heat. The Qi Standard has a protocol that can identify whether power is being absorbed by a foreign object so it can shut the charger off before overheating becomes an issue – that protocol is called Foreign Object Detection (or FOD).

When transmitting power through a multi-coil system, FOD becomes much more difficult. It’s important to address FOD as a top priority: if this protocol isn’t implemented properly, the charger can damage devices or random objects that rest on top of the pad, and can overheat to dangerous temperatures.

In a free-position system, you must implement both the Qi Standard’s methods and additional methods for FOD that are effective, reliable, and safe.

6. Eliminating Cross Talk and Interference

In a free-position system that supports multiple devices, there are multiple magnetic fields being formed on the same magnetic domain. Simply put, a magnetic field is formed for each charging device, and when there are multiple fields present, they can interfere with and intercept one another.

If crosstalk and interferences aren’t dealt with properly, the charging experience can be very unstable. Devices may stop and start charging rapidly and unpredictably – a non-starter experience for a consumer product.

7. Building a Chipset That Can Implement All of This

There are many complexities to consider within a free-position system, a few of which have been outlined above. In order to solve these issues and provide a quality user experience, a variety of complex algorithms need to be implemented. These algorithms all need to be integrated into a single controlling chipset.

An off-the-shelf chipset that adheres to the standard Qi functionality is unfit to power a free-position system reliably. A chipset that is uniquely dedicated to free-position charging is therefore required. Until very recently, before companies like Aira began to innovate in this space, that type of chipset had never existed for commercial use.

8. Coupling Hardware and Firmware

Apple has shown that there are advantages to engineering both the hardware and software of a given technology. When both are engineered by the same company, every component can be optimized to work as harmoniously as possible and therefore maximize the user experience.

The same is true for free-position wireless charging. The hardware and software must be coupled together. Every component matters and every detail needs to be accounted for: the size of the coils, how far apart they are from one another, whether they are on a lower or higher level of the matrix, how they get energized, etc. These are details that need to be precisely accounted for and understood in-depth within the chipset.

In the Qi Standard landscape, any chipset can be purchased and integrated into a wireless charger as long as general specs are followed. With free-position technology, the wireless charger’s chipset and hardware design must be engineered together.

9. Taking the “Intentional Radiator” Classification Into Account

The FCC classifies wireless chargers as intentional radiators (Part 15 Class C) – a fair description for systems that intentionally radiate power through magnetic fields. When designing a single-coil wireless charger, the challenges don’t extend too far beyond the ordinary FCC compliance hurdles related to electronics design.

However, as per its name, a multi-coil system groups together a large quantity of coils. From a physics standpoint, those coils are not much different than antennas. FCC compliant design becomes difficult when a system is packed with many transmitting antennas because it emits more radiation.

10. Working With Small Devices

As mentioned above, according to the Qi Standard, the most efficient wireless charging occurs when a transmitting coil is matched in size/spec with a receiving coil. The standard smartphone receiving coil is 40mm to 50mm wide, which means the transmitting coil should be about that size.

Now, consider the size of smaller devices on the market, like AirPods or Galaxy Buds. These receiving coils can be significantly narrower, measuring even less than 20mm across. When those small receiving coils are directly aligned with larger transmitting coils, they’re within an acceptable zone. However, when those small receiving coils shift off-center and become misaligned, they are very difficult to detect and charge.

When a free-position system is designed properly, the user should be able to drop any device anywhere on the charger’s surface. If they can’t, it’s not a true free-position experience. In order to successfully deliver power to small devices, a blended approach of hardware and software is required to detect the receiving coil and position the magnetic fields below it.

11. Manufacturing in a Way That Is Cost Effective

After each of the problems above is addressed, it will finally be time to get the system manufactured. In a high-volume manufacturing environment, every penny counts – however, many of the problems associated with free-position wireless charging require a hardware fix. Oftentimes, those fixes need to be multiplied by the number of coils in the matrix. As a hypothetical example, in a 16-18 coil system, a $0.05 coil fix can become an $0.80 to $0.90 fix. Those numbers add up quickly, and that doesn’t account for the larger product housing, packaging, and relatively high-output power supply.

Following standard retail pricing models, to calculate your MSRP, you need to multiply the cost of manufacturing a product by 4 or 5 times. That multiple is the only way all of the players involved will make enough profit to sustainably operate their businesses. Going back to that example, a $0.05 fix that becomes a $0.90 system fix will actually add $4 to $5 at retail.


Wireless charging is about convenience: the user should be able to drop their device on a charger without a second thought, in any position, and receive power. But before companies can give users the ultimate free-position experience, there are a number of design and engineering challenges that need to be addressed. Many of these challenges require solutions that can only be discovered along the path less traveled.

Single-coil wireless charging has become commonplace over the past few decades, and the technology is widely understood. Although it seems like a simple fix to place a few extra coils next to one another and widen the active area, multi-coil, multi-device systems present a wide variety of new problems and significantly exacerbate the existing ones.

Fortunately, there are a few forward-thinking companies that have spent many years working towards a fix for these problems, Aira being one of them. You can expect our technology’s imminent release – a wireless charging revolution that will turn the page on the industry as we know it.

About FreePower

In 2017, FreePower (FKA Aira) took the first steps towards reinventing a technology that had always fallen short of expectations: wireless charging. Today, we have more than 230 patent assets for our advanced system, all of which were engineered in-house. After years of perfecting the technology, we launched the second-generation FreePower with Tesla in 2023, followed by our latest innovation: FreePower for Countertop. From bar tops at a restaurant to kitchen islands and home offices, this embedded technology seamlessly integrates into surfaces of all kinds. We are now partnering with leading architects, designers, builders, and fabricators to bring power to the spaces that connect us.

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