“For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled” — Richard Feynman
Our previous post was about client power, the first key specification of wireless power systems. Client power is the useful power actually delivered to the client device, as opposed to other metrics that look good only on paper.
In this post we will look at the other half of the story and compare the two main wireless power technologies.
Have you ever installed a brand new Wi-Fi router, only to discover the signal doesn’t reach the basement? Or watched a YouTube video, having it come to a halt as soon as you entered a staircase?
In these areas the coverage wasn’t good enough. You were either too far from the router, or had too many obstacles in between. With wireless power, the situation is similar.
Coverage
We will use a simple drawing to illustrate coverage. The green circle at the top is the wireless power transmitter.
What would the ideal coverage be? Green everywhere!
This is of course too good to be true, so let’s talk about real systems.
Real-world Coverage
First, power doesn’t go in all directions. The cone shown below is a good approximation.
But most importantly, is the power level the same across the green area?
Let’s compare the two main methods to implement wireless power: Infra-red light, used by Wi-Charge, and RF radio waves.
The bottom line:
Infra-red wireless power maintains its strength all across the coverage area.
RF based power decays extremely fast with distance.
The above is not a result of bad engineering, but a fundamental property of nature.
Radio Waves — The Great and the Ugly
Radio waves are amazing. They are used in cellular, Wi-Fi, Bluetooth, AM/FM broadcasting and more.
Radio waves are amazing because they get everywhere. They radiate in all directions, they pass through walls, and they bounce from wall to wall. Remember the “all green” drawing? That’s how life is for radio.
What’s the catch? If you transmit everywhere, very little of what you transmit actually gets somewhere specific. Imagine collecting water by running around in the rain with a bucket…
For communication systems, this works! The signal transmitted by a cell tower can weaken by as much as 10 billion when received by a phone, and your cat video will still run flawlessly.
That doesn’t work for power. Today’s fastest phone chargers get your battery from empty to full in about an hour and a half. Add just one 10x loss and they are unusable.
To illustrate, let’s look at a $15 NORDMÄRKE Qi charger from Ikea:
It is small, thin, and is powered from a simple USB cable. It delivers 5W of client power while consuming double that (5V over 2A is 10W).
Now, let’s look at the $55 SJÖMÄRKE model:
The SJÖMÄRKE is designed to be installed below a desk, which is pretty cool.
But note how big it is, and how it requires a large, dedicated power supply:
Let’s take a look at its specs:
The client power is the same 5W, but the input power is 16.8W, almost 70% greater. Why is that?
You guessed it: The thickness of the desk. Just one half of an inch is enough to require 70% more power, a much bigger product, and have it cost almost 4x more.
That’s what half an inch does. Imagine what happens when you beam radio wave power across a room.
Light
Unlike radio waves, light can be aimed to point only at the wireless power receiver. Instead of running around in the rain you can use a hose!
Incredibly, having the energy pointed only at the receiver is a double win.
First, the huge efficiency improvement means that you can transmit power all across a room rather than losing most of it after the first couple inches.
Second, you no longer bathe the room in radiation. The wireless power energy passes through a narrow beam between the transmitter and receiver, and only there.
This is how this looks in practice:
The light green cone is the coverage of the Wi-Charge infra-red wireless power system. However, at any given moment the energy exists only in the dark green line, with exactly zero energy elsewhere. The system is smart enough to steer the beam exactly where it is needed, and only there.
In summary, coverage is the second key specification of wireless power systems. It describes the area being covered, but also how much client power degrades with distance.
Wi-Charge’s infra-red wireless power brings a wide coverage and uniform power delivery all across it. Beware of systems that specify coverage alone, without showing how client power varies with distance.
