Can wireless charging become more energy efficient?

Question

I've stumbled upon a few articles saying that wireless charging takes more energy to charge something compared to the cable. Like this one.

Like in the test from the article, it takes 47.33 % more energy.

Why is that? Is there something fundamentally wrong with the principle of inductive charging?

Or is it may be the current technology is just inefficient by itself and there is a room for improvement, so that it becomes just as efficient as cable charging if not more?

If engineers were to be improving the wireless charging technology, what are the directions to be looking at?

Answer 1

Wireless chargers involve large circulating currents in coils, running at relatively high frequency, and that is always going to be lossy.

Between skin effect, relatively low mutual coupling and the large circulating currents required to couple a reasonable amount of charge it just sucks compared to using a wire.

Can you make it more efficient? Sure, better capacitors, better litz wire, better rectifiers and maybe bigger loop areas on both sides allowing lower operating frequencies, but it all costs way more money or makes the device bigger, and those are not tradeoffs the market will accept.

A direct copper circuit is almost loss free in comparison, and that wireless charging is even that efficient is frankly stunningly good given the constraints of miss aligned air cored gapped coils driven by cheap electronics. You could maybe do better, a nice ferrite slug or one made of electrical steel to concentrate the fields would probably work (At the cost of weight and making the phone thicker) as would better litz wire or higher Q resonating caps, but it costs money and screws up the form factor.

Answer 2

• You never transmit through an ideal RF circuit and ideal antenna with 100% of the energy radiated through the antenna. Some manner of bandpass filter is required to ensure that the transmitter is only transmitting at the intended frequency band.
• The energy does not travel through an ideal medium without losses. Air has much higher losses than a copper conductor.
• The energy is not 100% directed into the receiver. It will "spill" all over the place.
• The receiving side does not catch 100% of the energy received and does not have an ideal antenna/RF circuit either.
• The receiver may also have to filter the energy down to a certain frequency band, so that it only charges from energy transmitted at certain frequencies. Or otherwise it might trigger unintentional charging from other sources when the product is in use.
• Further losses happen as part of the charging. The charge current has to be controlled, the charger might have to measure temperatures etc. On a wired charger, it can just stop charging when the battery has the wrong temperature. I'm not sure how this works in practice in a wireless charger. Suppose you place a freezing cold phone on a charger plate - it will not charge until the battery is warm enough to charge without taking damage. How does the transmitter know that the battery is cold and stop wasting energy?

Where exactly along this path most of the losses occur, I don't know. But several of the above bullets are regulated by the laws of physics, so I doubt a wireless charger will ever get near as efficient as a wired one.

Another big problem is probably user impatience. Everything has to charge so darn fast and at the same speed as wired chargers, meaning relatively huge currents.

On the flip side, an inductive charger could at least in theory pick up EMI from other sources and use that for charging. It would be neat if you could charge your phone from all wasteful radio energy floating around in the ether anyway, but I suspect that EMC and radio legislation makes that quite problematic. As well as them impatient users - charging a phone solely from the radiations from its own phone mast would take a long while.