EV Charging 106: Wireless Charging

Wireless charging

In all the articles before, we mostly talked about charging EV batteries with wired connections. It is possible to charge an EV without any wires. This can be done when the vehicle is parked, i.e. stationary charging, or when the vehicle is moving i.e. dynamic charging. As you would expect it, wireless charging makes EV charging quite easy. There is not much the driver needs to do other than park or drive over a charger. There is no need to plug anything, and you can pretty much automate the charging process. To understand wireless charging we need to understand how wireless power transfer works.

Wireless Power Transfer

Wireless Power Transfer (WPT) is not a new concept in electricity. In fact, Nicolai Tesla, the father of Alternating Current (AC) was experimenting with wireless power transfer in 1890's. Actually, there are not many design changes between the 'Tesla' coil and any modern wireless EV charging systems. Any wireless power transfer system has two parts a transmitter coil and a receiving coil, which are magnetically coupled. So for wireless charging, the EVs need to have receiving systems for the AC power. We also need a transmitting system, which could be in the form of a charging pad as shown in Figure 1. Alternately, the transmitters could be underground, embedded inside the roads as shown in Figure 2.

Figure 1: Wireless EV charging pad for an electric car

Only High-frequency AC power

One important concept to remember here is that only AC power can be transferred wirelessly. That is because without AC power we cannot achieve induction or magnetic resonance- which are the most common ways of wireless power transfer. In the case of induction, the magnetic field created by the transmitter induces a current in the receiver. The difference between both of them is that while induction can happen at any frequency, magnetic resonance only happens at a unique frequency. Magnetic resonance is a special type induction that happens when two magnetic fields come in synchronism at the resonance frequency. The latter is important because the efficiency of power transfer improves at magnetic resonance. Even, Nicolai Tesla's wireless power coils based on magnetic resonance.

Let's not forget that the battery only accepts DC power, and wireless power is AC. So for charging the EV, the AC power still needs to be converted to DC power. And the converters which do this are part of the on-board chargers for the vehicles. So there is no circumventing of the charger to achieve wireless power transfer. So for wireless charging, the EVs should have both receivers and chargers on-board. The limitation of the maximum power for this EV will be the capacity of the on-board charger. One caveat here is that weight of the onboard charger has an implication of total propulsion efficiency. As charger power increases onboard chargers get heavier and additional weight is not an efficient design.

Did you know that the first patent for wireless charging of EVs was filed in 1894?

The second thing to remember is that the regular AC power, which has a frequency of 50/60 Hertz is not ideal for wireless power transfer. To obtain better efficiencies the regular AC is converted into high-frequency AC power first, and then fed to the transmitting system for the wireless power transfer. For another possible method for wireless charging, i.e. using capacitive power transfer high-frequency AC is used. Typically the frequencies could be 20,60 or 85 kHz. This also brings us to the primary problems with WPT, the efficiency problem, and electromagnetic inference from the high-frequency waves, which we will examine one by one.

Figure 2: Wireless charging facility for EVs with underground transmitters

Electro-magnetic Inference

There is a lot of concerns about the strong electromagnetic fields generated by the wireless charging and the health risks of non-ionizing radiation from these sources. There are strict guidelines surrounding the frequencies that can be used for EV charging to prevent its interference with the other telecommunication systems. Shields are also deployed along with chargers to prevent unnecessary issues. While the majority of problems are expected to be solved with commercialization, it should be kept in mind that wireless charging is impossible without high power radiation.

Efficiency

The trailblazers in wireless charging research, who put the technology on the map are researchers from the University of California, Berkeley. In their famous project called PATH, they created a roadway to charge electric buses, thus providing a proof of concept for the technology. One of the key challenges with the WPT has been its low efficiency. In comparison with the plugin charging (90-95%), the efficiency of the wireless chargers was quite low (60-70%). Lucky for us, there are a lot of researchers who are dedicated to solving this problem. One of the main reasons for the problem was the losses between the transmitter and the receiver.

The researchers from KAIST University in Korea were exceptionally important for increasing the efficiencies to beyond 90%. They were also responsible for developing the popular high power dynamic charging system for electric buses. Under a very expensive project called OLEV (On-Line Electric Vehicle), they developed a system which can charge buses when they are in motion, and when they are stopped. A schematic of the system is below in Figure 3, and you can see that system had road embedded power lines. When the vehicle's receiver gets aligned properly, that particular transmitter segment will come on, and charges the battery.

Figure 3: Schematic of a dynamic wireless charging system

Static or Dynamic

When we talk about efficiencies, there is a case to look into the total efficiency of the propulsion system. The main issue with the EV batteries is that as the battery size increases, the batteries get heavier. Weight is an important parameter for an efficient vehicle propulsion system design, and the most efficient design has the least weight. With wireless charging, it is possible to design vehicle drive trains in the future with smaller battery packs. Thus, dynamic charging can help lower the cost of EVs by reducing the size of battery packs. With road embedded power lines, such systems can enable opportunity charging for EVs as they drive. However, today these dynamic charging systems are pretty expensive.

With dynamic charging there could be a future where electric vehicles never have to stop for charging

On the other hand, static charging systems are less expensive. They becoming popular as convenient charging options vehicles. There is no shock safety hazard for the driver as there are no wires to plug or unplug. There are many companies like WiTricity, Plugless, and Momentum dynamics who are making these really cool chargers for cars and buses. These chargers are revolutionizing charging at parking lots and garages. A link to wireless charging of electric car in a garage is here:

Link to other articles in this series:

Link to Part 1: EV Charging 101

Link to Part 2: EV Charging 102

Link to Part 3: EV charging 103

Link to Part 4: EV Charging 104

Link to Part 5: EV Charging 105

Note: I am an electrical and electronics engineer and not an expert in EV or batteries. However, my current area of research is EV charging, and my auto-didactic tendencies are lately on overdrive with my focused reading. The article is a summary of my notes from multiple sources. A fellow EV enthusiast may find it useful, though caution is advised. As this article intended to help me in improving my writing skills, please do let me know how I am doing in the comments section.