A close-up view of the inside of a multi-coil wireless charger
A close-up view of the inside of a multi-coil wireless charger

Why more coils does not always mean better charging

By Trent

One of the key challenges in achieving effective wireless charging is positioning the coils of the charger and receiver close enough to one another that they can transfer power. Incorrect positioning can lead to slow charging or in some cases no charging at all. In order to solve this problem, some manufacturers have attempted to squeeze several coils into a single charging surface with the hope that the user will be able to find the sweet spot on at least one of those coils in order to commence charging. While this appears to be a creative solution, the use of such arrays may not deliver the most efficient charging experience and the use of a single enhanced coil may in fact be a better way to deliver worry-free wireless charging. 

Wireless charging and coils

On a very basic level, wireless charging works by positioning two coils close enough to each other (usually one in a charger and another in a receiver such as a smartphone) that they are able to transfer power by magnetic induction without physically touching. Depending on the engineering quality of the devices being used, the separation between the two coils needs to be as little as a couple of millimeters in order to charge. The fact that these coils are usually concealed behind covers or casings can make it very difficult to find the correct positioning, with the result often being slow charging or none at all.

An illustration of how coil positioning can affect wireless charging for smartphonesBoth the transmitter (charger) and receiver (phone) contain charging coils which must align correctly in order to transfer power.

If you're new to the world of wireless power transmission, check out our Ultimate Guide to Wireless Charging to get the low-down on all aspects of this exciting charging technology. 

Multiple coils don't increase charging power

Probably the most common misconception to address is that even in an array of multiple charging coils, only one of those will be active at any one time. It is not possible to pool the energy of several coils together to deliver an overall higher dose of charging power to the receiver device. The use of coil arrays is primarily in the interests of convenience and efficiency improvement rather than overall power delivery.

An infographic showing the layout of a single coil and multi-coil arrayCoils are usually layered on top of one another in order to achieve a multi-coil array.

"It is not possible to pool the energy of several coils together to deliver an overall higher dose of charging power"

More coils can create more problems

Some manufacturers believe that there's more likelihood of lining up with at least one coil if there are more of them. This is indeed an innovative solution which can increase the chances of correct coil positioning without needing to pay a great deal of attention to where on the charging pad you may be placing your device, but the implementation itself brings about two further issues which can negatively affect charging performance.

Layering of coils increases distance - In order to fit the maximum number of coils under a charging surface, these coils are often layered, partially overlapping each other. Those coils installed on the lower layer(s) of the array will therefore be further away from the receiver device's coil, reducing the efficiency of power transfer and resulting in slower charging or excessive power consumption (or both).

Obstruction of overlapping coils - Another side-effect of layering is that the coils in lower layers can be partially obstructed by those above, which combined with the aforementioned distance increase for bottom-layer coils can further decrease the efficiency of power transfer.

An illustration of how multiple coil layers can cause obstruction of wireless charging powerThe lower coils in an array are located further from the charging surface, and can be obstructed by other coils.

Added components, reduced quality

It goes without saying that adding duplicates of the same component to a product will increase the overall cost of production, and with the charging coil being one of the more advanced components inside the average wireless charger, adding several to a single unit will quickly increase the price paid by the end customer. As multi-coil arrays do not deliver any significant performance improvements in terms of charging output or speed, paying significantly more for just the convenience of device placement may be a hard sell for many customers. In order to keep production costs down, manufacturers may choose to use several lower-spec (and lower-cost) coils instead of a single high-performance one, and as only one coil is ever in use at a time, the overall charging experience may suffer due to the decreased performance of each single coil.

Making one coil better

The alternative to implementing large arrays of inferior coils is to engineer a single charging coil which is capable of charging over a larger area, thus reducing the need for pinpoint accuracy when placing a device down on the charging surface. When using only one coil, a greater investment can be made in design and materials to maximize the effective charging area of the coil as well as improve output efficiency and cooling for better overall charging performance. This was the path favored by Moshi when we developed our Q-coil™ module, which you can read more about in our dedicated post.

"When using only one coil, a greater investment can be made in design and materials"

While there are some advantages of utilizing coil arrays to improve wireless charging, the additional challenges which arise in the use of such a design can further affect the user experience. Moshi's preference for premium, single-coil charging solutions has led to the creation of some of the industry's most awarded wireless chargers, with large effective charging areas and enhanced passive cooling delivering improved performance. Learn more about our range of Q Collection wireless chargers at moshi.com.

 

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