By Julian Dodman
Wireless Charging, Great….. Right?!
Or is it just a marketers fantasy…?
Modern technology is brilliant! We have Gb/s optical cables, full duplex data transmission, Quantum Dot LED displays, the list goes on.
But there’s one marvel that really ruffles my feathers… wireless charging in mobile phones. It’s made out to be this amazing cable-free way for you to charge your phone, but is it really that great? Let’s find out!
Okay, first let me explain a little bit about how wireless power transmission works. As you may already know, electricity is the flow of electrons through a conductor (or rather, the electrons bumping into each other like a Newton’s cradle).
Electrical flow or current (remember this for later) is called an Ampere which you might be familiar with!
So, now you know about electrical current, let’s start talking about electromagnetism. When current flows through a conductor, the charged particles create magnetic fields around that conductor. The strength of this magnetic field is proportional to the current flow, so larger current = stronger field.
The electrical field created by a single line of wire is almost imperceivable, which is why inductors exist (an inductor is just a coil of wire). When current flows through an inductor, the magnetic fields converge and ultimately result in a larger field.
Fields created from an inductor can interfere with other circuits and wires nearby, for example, when current flows through a wire, the fields induce current in the wire right next to it. (Incidentally, this is how transformers work!)
These fields inducing current in nearby conductors is how wireless power transmission works, there’s just one problem, direct current (DC) magnetic fields are incredibly weak which means you’d need to pass hundreds of Amps through your inductor to power anything….
This is where alternating current (AC) comes in. I won’t bore you with all of the maths and theory of AC, instead, all you need to know is that it travels back and forth through the conductor at a given frequency (Which is 50-60 Hertz in most countries).
So what happens if we pass this AC though our inductor? Because it’s essentially traveling through the inductor nearly 50 times a second, we get an exponentially stronger field!
Right, now we’re talking! With our AC inductor, we can place another inductor somewhere near it and power stuff, excellent!
There’s just a teeny tiny problem… if we plug any AC signal into our phone it will instantly become a firework.
So, the way wireless phone chargers do it, is they use a transformer to step the mains AC down to a lower voltage (somewhere around 5-10V), use that lower voltage to power our first inductor. This means that our second inductor (the one in the phone), will have 5-10V AC induced into it.
Then they use a rectifier to convert the nasty AC into 5V DC which is safe for the phone, and won’t bring about New Years celebrations early!
Well… now you know how wireless power transmission works, why don’t I like it?
The answer is power loss… power loss is a problem that engineers have to work around every day. Electrical circuits aren’t perfect, and some power is lost, either in heat or electrical fields.
Most circuits in your house/boat will probably have an efficiency of around 80%, which is pretty good in today’s standards. This means that about 20% of the power going into the circuit is lost in heat or electrical fields.
Before I go any further, I want to mention a little someone called Nikola Tesla… I think you might have heard of him. He’s only responsible for most of the tech we use today!
You’ve probably heard of the Tesla Tower or Wardenclyffe Tower, Nikola Tesla wanted to use this tower to transmit power wirelessly across the world (using resonant frequencies and electromagnetism)!
Sound familiar? This is the exact same process that our two inductors are doing!
So why are we not powering anything wirelessly, can anyone tell me…?
Yes, you at the back there! Correct, power loss. This process has a lot of power loss, most of it is lost in the environment (through capacitances to ground and such). This means that you can only really power anything meaningful a few tens of metres from the tower.
So back to our wireless phone charger. Because of power loss, you have to charge it a few centimetres from the transmitter… surely that defeats the point of it being wireless?!
You have to set the phone down on the transmitter… why don’t you just plug it in instead!
Not only that, but there’s even more power losses (yep, more). Converting the AC to transmit it, and converting down to DC to the phone, this process wastes power as well.
So overall, wireless phone charging is around 30% less efficient than plugging your phone in directly (especially with the new USB-C Power Delivery (that’s a topic for another piece!)), and you don’t gain any maneuverability out of it!
My conclusion is that some marketer somewhere has gone, you know what would be cool, wireless charging. Hey, engineers, make our phones wirelessly charge!.
Save your electricity bill / boat batteries… don’t support wireless charging.