Implementing wireless electric vehicle charging

Almost all EVs today use conductive charging, but there are lingering concerns. Safety is a potential issue, especially in wet conditions. Home-based 110V or 220V systems take up to 10 hours to fully recharge an EV. Public fast-charging stations have more power available and can charge EVs in much less time, but they take up large amounts of space, and the equipment can be stolen. Also, fast chargers can degrade battery life.

Wireless charging has been around for years in low-power consumer applications – wireless shavers are widely available, and electric toothbrushes have used it since the early 1990s. Even the medical field is using wireless charging for subcutaneous implants.

In automotive, wireless charging was a feature of the GM EV1, the first mass-produced EV from a major manufacturer. The EV1, as well as a few hundred first -generation Toyota RAV4 EVs and Chevrolet S10 EVs, charged via induction using a paddle conforming to the J1773 standard.

There were three charging levels. The vehicle itself included a 1.2kW level 1 charger, which ran off a standard 120V outlet. It could provide a full charge, but took as long as 14 hours; it was primarily intended to provide a quick charge to get the vehicle home or to a commercial charging station.

For home installation, a level 2 charger provided 6.6kW but required a 208-240vac supply. Recharging the EV1 to full capacity took as long as eight hours, although it could achieve 80% charge in one to three hours.

Unfortunately for J1773, in 2001 the California Air Resources Board (CARB) decided on J1772, a conductive (wired) charging interface, as the standard for California EVs, causing its demise.

After that, progress in wireless charging has been glacial, even though electric vehicles have been a part of the mainstream automotive landscape since the introduction of the Toyota Prius in 1997.

Now, though, the future is looking brighter. In its research report, "Wireless Charging Systems for Electric Vehicles", Navigant Research forecasts that worldwide sales of wireless EV charging equipment for light-duty vehicles will grow by a compound annual growth rate (CAGR) of 108% from 2013 to 2022, achieving annual sales of slightly less than 302,000 units in 2022.

Let's not get too excited, though. To reach the quoted sales in 2022 with a CAGR of more than 100%, the forecast needs to begin from a minuscule starting point of 400 units in 2013, a year that saw around 110,000 sales of pure EVs.

Despite its slow progress in the market, wireless charging has several desirable features:
 • The charging process is simple, automatic and doesn't require driver input
 • Compared to a wired system, it's resistant to vandalism and can even be installed underneath the garage surface
 • No contact is required, and there are no exposed electric connections
 • The impact on the vehicle is low – the installed equipment is small and light
 • New designs have high efficiency, comparable to that of wired charging

Principles of wireless charging
Wireless charging for EVs uses near-field charging (NFC): a transmitting coil produces a magnetic field that transfers energy via induction to a nearby receiving coil. The fraction of the magnetic flux generated by the transmitter coil that penetrates the receiver coil and contributes to the power transfer is a function of the distance between the two coils. The transfer efficiency depends on the coupling (k) between the coils and their quality factor (Q).

Figure 1: WPT system block diagram (source: Witricity)

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