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Implementing wireless electric vehicle charging

Posted: 30 Dec 2015     Print Version  Bookmark and Share

Keywords:wireless charger  electric vehicles  EV  J1773  J2954 

Qualcomm's Halo WEVC technology uses such an approach. Tests have shown that a distance of 150mm (6 inches) provides the best efficiency, but a maximum height of 250mm (10 inches) is possible, allowing SUVs to use the wireless charging pads; operation at greater height will be needed to install pads under driveways or road surfaces in the future.

Evatran offers their Plugless L2 3.3kW static charging system for sale or lease direct to consumers. The system runs on 208 – 240 VAC residential power and includes an interlock to prevent simultaneous inductive and conductive charging. Initially, versions are available for the Chevrolet Volt, the Nissan Leaf, and Cadillac's ELR.

Figure 3: A Plugless under-vehicle adapter (source: Plugless Power)

Witricity, working with resonant coupling IP developed at MIT, has announced the WiT-3300 development kit to help users evaluate their WPT technology. The company claims a power transfer of up to 3.3kW and coil-to-coil efficiency of up to 97% over a distance of up to 15 cm. Available options include testing software, a resonator test bench, and other evaluation items.

What of the OEMs themselves? They're waiting for inductive charging standards to be finalized before taking the plunge, although Infiniti did include WPT in their LE concept vehicle that they exhibited at the New York Auto Show in 2012.

Other OEMs are conducting feasibility studies and trials. In 2014, for example, Toyota announced that it was conducting verification testing of its 2kW wireless charging system. To help the driver park in an optimum charging position, the test vehicles include a parking assist function that indicates the position of the transmitting coil in the parking space.

In motorsports, although Formula E, the open-wheel racing series for electric vehicles, doesn't yet use wireless charging for race cars, its modified BMW i8 and i3 course cars stay charged with the Qualcomm Halo system; one car is located at each end of the pit lane to sprint to on-track incidents when needed.

Safety concerns
There are two main safety concerns with wireless systems: foreign objects between the coils, and the effects of EMI.

A metallic object between the transmitter and receiver coils can be heated by the magnetic field. Above a certain size, the heat generated poses a safety risk, One way to detect its presence is by the disturbance in the magnetic field caused by induced eddy currents, which can be detected by a sensor array. The WiTricity Wit-3300 system uses just such an approach; if it detects a potentially hazardous object, it shuts down the WiTricity power source via the CAN serial bus. Metallic objects less than 2.5 cm2 do not pose a risk and are not detected.

The other concern is EMI. Careful coil design minimises the amount of EM radiation that emanates outside the vehicle envelope, so even lying on the ground up against the vehicle does not heat tissue or pose an increased risk of cancer. In equipment, there is concern that wireless charging might interfere with the operation of other wireless equipment such as remote keyless entry (RKE) systems.

Looking to the future
Static WPT systems will slowly make their appearance over the next few years as aftermarket systems become more widely available. The pace will accelerate once J2954 is finalized, and OEMs begin incorporating WPT as a standard feature. Longer term, the charging infrastructure will slowly expand, with both stand-alone installations and as additions to gas stations.

One method being investigated is dynamic charging, where an EV recharges in quick bursts from distributed chargers during its daily travels. A partially dynamic charging system has undergone trials in buses; the bus picks up energy from when it pauses for thirty seconds or so each stop. Adding WPT allows the bus to have smaller batteries, reducing size, weight and cost.

Looking forward a decade or two, the really big change will be the implementation of fully dynamic charging, where EVs pick up power as they pass over coils embedded at intervals under the road surface.

About the author
Paul Pickering is a consultant who has over 35 years of engineering and marketing experience in the electronics industry, including time spent in automotive electronics, precision analogue, power semiconductors, flight simulation and robotics. Originally from the North-East of England, he has lived and worked in Europe, the US, and Japan. He has hands-on experience in both digital and analogue circuit design, embedded software, and Web technologies. He has a B.Sc. (Hons) in Physics & Electronics from Royal Holloway College, University of London, and has done graduate work at Tulsa University. In his spare time he plays and teaches the guitar in the Phoenix, Arizona area.

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