Wireless phone charging may feel modern, but the underlying technology was invented by Nikola Tesla in the late 1800s. He used magnetic induction to power lamps wirelessly.
Wireless charging uses the same magnetic induction to wirelessly transfer energy between two coils. In the first coil, the transmitter, a magnetic field is generated with alternating current. Because the current changes constantly, the magnetic field also changes constantly. That field is picked up by the second coil, the receiver, in the phone. There, this magnetic field is converted into an alternating current via the reverse principle. That power is used to charge the phone battery.
But in this process, the phone does heat up. It gets much warmer than wired charging. How is that possible?
This is because much of the energy in the process is lost in heat, says Bart Smolders, professor of electromagnetism at Eindhoven University of Technology. The metal wires in the coils have a resistance. That resistance tries to stop the moving electrons. The electrons then collide with the atoms of the electric wire. In this collision, the kinetic energy of the electrons is partly converted into heat. The higher the current, the more heat is released. There are options to reduce that heating, according to Smolders. “You can make the wires in the coil thicker, so they have less resistance. But the tricky part is that in the case of a telephone, it should only take up very little space.” Another possibility is to use a material with a lower resistance, but that is more expensive.
In any case, the efficiency of wireless charging is lower than wired, says the professor. “You often just throw away 40 to 50 percent of the energy you put into it. That is a fundamental flaw of the system. The magnetic field that the transmitting coil generates radiates in all directions, even where the receiving coil is not located.” But you don’t feel that loss in heat.
Boil a pan of water
According to a test from technology website ZDNet heats up a phone to about 35°C during wireless charging. That is not a temperature at which the phone will be damaged. Although the temperature can be higher locally, Smolders emphasizes.
Can that residual heat also be usefully used, for example to boil a pan of water? “No, it concerns relatively small capacities,” says Smolders. To charge a phone you use about 10 to 20 watt-hours (36,000 to 72,000 joules). Suppose that 5 watt-hours of that go to heat in the telephone, that is 18,000 Joules. Heating a liter of water at 90 degrees will cost you more than twenty times as much energy. “Only in winter, when it’s cold, can you use the heat to heat your room a little bit.”
There is also a protection in the charging system to prevent energy from being delivered to other metal objects, foreign object detection that’s called. The two coils in the transmitter and receiver communicate with each other and compare the powers sent and received. If the difference between them is large, some of the energy is probably absorbed by another object. That can heat the other object. The energy supply then stops to prevent dangerous situations.
A version of this article also appeared in NRC Handelsblad on 20 November 2021
A version of this article also appeared in NRC in the morning of November 20, 2021