Why don't you charge your vehicle inductively?

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Actually, inductive charging sounds as if it would sell itself: Park the car, done, it all happens automatically. Manufacturers then used these advantages to woo the few automotive products that made it onto the market: Inductive charging is much more convenient than having to handle highly flammable, strong-smelling liquids at a filling station. In practice, however, the higher costs were only offset by minor advantages. A new positioning standard could at least provide impetus for fleet operations and public charging infrastructure.

How does inductive power transmission (roughly) work?

It has been known for almost 200 years that electrical energy can be transmitted wirelessly using coils. A current-carrying coil generates a magnetic field. The temporal change of such a magnetic field in turn induces a current flow in a conductor (law of induction). If alternating voltage is applied to a coil, the magnetic field changes with the oscillation of the alternating voltage. A second coil placed in this oscillating magnetic field therefore produces alternating voltage of the same frequency. The voltage ratio can be controlled by varying the number of windings, which is the principle of the transformer.

In inductive charging systems for cars, the alternating voltage usually has frequencies in the range of 85 kHz. This is a good compromise between efficiency (which increases with higher frequencies) and the necessary electronics (which become more complex and power-hungry with increasing frequencies). The distance between transmitter and receiver should be as small as possible if the losses are to remain low. This is why there are systems in which the air gap is mechanically reduced: Either the coils in the floor are raised or those in the vehicle are lowered. Of course, this type of mechanism only makes sense for stationary energy transmission. In the "Emil" project, for example, the receiver coils of the electric bus were lowered towards the ground at the bus stops, which charged the bus with 200 kW. Since the magnetic field in metallic objects such as a wrench generates currents and can heat them up like pots on an induction stove, high-power induction systems for vehicles require object detection. The control unit normally detects such foreign objects by changing a weaker test field that it sets up before the charging process.

Losses compared to the cable

Current inductive charging systems for electric vehicles have achieved losses of eight percent or less. However, the losses of conductive charging (cable losses) stated by manufacturers for comparison are often unrealistically high; Mahle, for example, states six percent over cable. According to DIN VDE 0100, a maximum loss of four percent is permitted in a complete building installation for safety reasons, and most of this is also incurred by the inductive charging station because it only transmits a few centimeters wirelessly. The cable losses with the recommended 2.5 mm² cable cross-section for 16 A current are negligible over the replaced lengths (additional length of type 2 cable required). For Mahles' calculation to work out, the plugs would have to be extremely frayed. Although conductive transmission is much more efficient, the losses for inductive transmission are still in a range that is perfectly practical. The only question is how much advantage it brings for its additional costs.

Early examples

Inductive power supply has long had its place, for example in the electric battery toothbrush, which can then be glued waterproof, or the Qi charging cradle for smartphones. However, it was also used early on in electric car construction. The GM EV1, for example, which was offered for leasing from 1996 and later became the star of a tendentious documentary, charged inductively. GM designed a paddle with a coil in it that was inserted into a slot in the front of the EV1, which contained the receiver coils.

By eliminating open contacts, GM provided greater safety and also better weather protection for the charging solution. I think about it every time I have to fumble this rubber cover off a Type 2 plug.

For German manufacturers, it was the plug-in hybrid, among other things, that inspired manufacturers to develop inductive charging solutions. It was already clear that a small battery meant more frequent plugging in, and the technology could then also be used for fully battery-electric cars. Mercedes-Benz tested inductive charging from 2014 with Qualcomm's "Halo" system, which could deliver up to 22 kW like a wallbox. The inductive charging set was to be offered as an accessory for the S 500e in 2017, but never went on sale. In 2019, the US company Witricity bought the technology from Qualcomm. For a long time, Audi's press department wrote about "Audi Wireless Charging" (AWC) in the brochures for near-production concept cars. The system shown worked with a base plate that moved upwards to reduce the air gap and thus increase efficiency. AWC never appeared in series production. The Fraunhofer Institute designed a solution in which the receiver coils were located behind the license plate, which had to be used to kiss a vertical transmitter column (the transmitter column yielded slightly accordingly). The system was intended to be retrofittable, but was not even included in the original equipment.

At BMW, on the other hand, there was something real to buy in 2017: the "CarPad" module for the car as a leasing option for 890 euros, along with the associated "GroundPad" for 2315 euros. Both Audi's and BMW's systems were designed for a single-phase connection to 230 V with 16 A current (i.e. 3.6 kW gross, BMW stated 3.3 kW net). Although BMW's pads made it to some customers, this intermezzo was also short-lived: the option disappeared after less than a year. The solutions suffered from the fact that the additional convenience of inductive charging was not worth the extra cost to customers - least of all in the case of the plug-in hybrid, which, contrary to its name, was practically never plugged in anyway, especially at that time with the even smaller batteries in fleet operation. In the case of BMW and Audi, there was also the fact that the systems were proprietary and also designed for the low gross single-phase output of 3.6 kW. A system that can establish itself on a broad scale needs at least the power of a wallbox (i.e. at least up to eleven kW) and also its compatibility. Or would you want to buy a new wallbox for every new car?

Standards are set

The necessary standardization can be found in the SAE standard J2954. Over the years, some manufacturers have not been entirely satisfied with this standard because it reflects the technology of the leading manufacturer WiTricity. However, there were no comparable alternatives. Future standardized solutions are therefore likely to follow J2954. The aim of the standard is what is usual for power plugs: the interface must be such that an inductive receiver (analogous to a plug) can be developed independently of a transmitter (analogous to a socket). Only then would it be guaranteed that a charging loop in the garage can also charge the new car of another brand if both follow the same standard. As with the type 2 plug, the energy transfer is preceded by a small data handshake, in this case wirelessly of course.

Towards the end of 2023, a positioning solution developed by the German supplier Mahle, the "DIPS" ("Differential Inductive Positioning System"), was added to the standard. The solution is designed to enable the vehicle to align itself at an early stage through steering movements so that the transmitter and receiver coils are optimally aligned. In simple versions, this can be done by giving visual instructions to human drivers. However, positioning can also be automated relatively easily in more expensive vehicles with appropriate actuators. With DIPS, a vertical coil in the transmitter generates a horizontal magnetic field that the receiver in the car can recognize early on for rough positioning. Fine positioning is then carried out via four horizontal coils with their four vertical magnetic fields. Each of the four coils is given its own alternating current frequency to identify it.

Outlook: still no boom in sight

The positioning is intended as a minimum requirement for public charging stations and light commercial vehicles, particularly with regard to automated driving. Solutions for private cars could omit this part of the standard, and in general the extension of the standard is unlikely to trigger a wave of products there anyway. In fleet operations, on the other hand, a system with this level of automation could be worthwhile in niche markets. In conjunction with solutions such as remote-controlled automated parking, convenient automated charging parking lots are also conceivable as a business model. However, the extended standard will not bring the big inductive boom that the standardization committee has been promising for over ten years. After all, the much cheaper plug is not annoying enough for that.

(cgl)