Source : Mouser Electronics

As Electric Vehicles (EVs) become a more prevalent mode of transportation, discussions about different electric charging options are increasing. Wireless charging is one option that received a great deal of interest in the past few years. Proponents see wireless charging as a simpler, more convenient way to charge EVs because it eliminates the hassle of handling charging cables. Also, they envision charging vehicles at rest or in motion through charging pads built into roadways. A number of companies are actively testing systems—and even offering kits to retrofit EVs—for wireless charging. But, is wireless charging a viable option for EVs?

EV Wireless Charging Overview

The principle behind wireless charging is the same as the one used to wirelessly charge low-power devices such as cell phones and electric toothbrushes. A primary coil built into the charging location creates a magnetic field when it is powered up. This magnetic field induces a current in a secondary coil attached to the battery of the device being charged. Wireless charging works best when the primary and secondary coils are at an optimum distance from one another.

Types of Wireless Charging

There are two types of wireless charging:

• One type of wireless charging is basic magnetic induction. In basic magnetic induction, the magnetic field in a primary coil induces a current in a secondary coil. These loosely coupled coils require precise alignment to achieve acceptable charging efficiencies.

• Another wireless charging method that has proven to be a much more efficient way to transfer power is called resonant-inductive coupling. In this case, both the primary and secondary coils use oscillating circuits that resonate at the same frequency to create precise impedance matching. The result is a magnetic field more narrowly confined to the space between the two coils, even when alignment is not perfect. Resonant-inductive coupling, or magnetic-resonance charging, is not only more efficient, but it is a better way to manage magnetic fields at the higher power levels required for EV charging.

In a typical EV wireless charging configuration, the primary coil is in a pad that sits under the car and plugs into the AC power. The secondary coil is located in the car. When parked, the coils properly align.

Who’s Developing EV Wireless Charging?

A number of companies are developing wireless EV charging technologies. These include:

• Several EV manufacturers are developing wireless charging prototypes and options for their cars. Nissan is developing a wireless charging option for the Leaf that would use autonomous parking features to precisely align the vehicle over its charging pad. BMW, Mercedes, and Tesla are also offering wireless charging options.

• WiTricity, a pioneer in magnetic-resonance charging, offers wireless charging solutions that charge at rates between 3.6kW and 11kW with 90 to 93 percent efficiency. Their system consists of a primary coil in a pad on the ground and a secondary coil attached to the underside of the vehicle. Several auto manufacturers are working with WiTricity to develop their wireless charging options.

• Qualcomm developed a similar wireless charging solution, called Halo, and they are developing technology that will charge EVs that are moving. Qualcomm successfully charged EVs traveling at 112.654kmph on their 100m test track with primary coils built into the road surface.

What are the Current EV Wireless Charging Challenges?

Though wireless charging holds great promise, wireless charging is not ready for prime time. Current barriers to widespread adoption include:

• Wireless charging is very expensive compared to equivalent residential EV charging stations.

• There are unknowns about the effects and safety of magnetic fields associated with wireless charging on a large scale.

• Today’s solutions offer very slow charging. The slow-charging aspect becomes even more of a disadvantage in larger EV batteries.

• The lack of wireless charging standards could limit the portability of wireless charging equipment—for example, a wireless charging parking space might only work on a specific type of car.

Christopher Michelbacher, EV Charging & Infrastructure Manager for Audi, sees the current challenges to wide adoption of wireless charging in this way. “The technology is not fully mature, and there are a number of cost and operational tradeoffs. At the present time, the best application may be for EV fleets rather than personally owned EVs. Some fleet operators are actually looking at that. In the case of vehicles that normally park at loading docks, alignment is not so much of a problem because the vehicle’s location is defined by the loading dock. The back of the vehicle touching the loading dock is the alignment mechanism. Also, some fleets have a scheduled route that allows you to know how much energy they will use between charges and how much time they need to charge. You also don’t have to worry about proprietary systems on different vehicles, because the same trucks will always be pulling up to the loading dock. It might work for some battery electric buses in a transit system where they run fixed routes, and they’ve got known stops or known break times where they can recharge in order to complete their route throughout the day.”

What’s Next for EV Wireless Charging?

Wireless charging is still evolving, but the possibilities are worth the effort. Technologies such as movable primary coils and new materials that make secondary coils lighter with lower wire resistance can make wireless charging solutions that are more efficient and work at higher power outputs. If wireless charging succeeds in becoming truly cost-effective, it would completely transform the way EVs are charged and even how they are used.

Key Points

• The most efficient way to wirelessly transfer power is through resonant inductive coupling, in which both the primary and secondary coils use oscillating circuits that resonate at the same frequency to create precise impedance matching.

• Qualcomm has successfully charged EVs traveling at 0.07kmph on their 100m test track with primary coils built into the road surface.

• The best application for wireless EV charging at the present time may be for EV fleets such as trucks that usually park at loading docks and have a scheduled route that makes their power consumption predictable.