SAE International on October 22, 2020 announced publication of the first global standard that specifies, in a single document, both the electric vehicle (EV) and EV supply equipment (EVSE) ground system requirements for wireless charging of electric vehicles. The new standard, SAE J2954, helps pave the way for charging without the need for plugging in — widely considered to be a key enabler for accelerating the adoption of EVs and autonomous vehicles.
The new standard was more than a decade in the making. SAE kicked off its pioneering pre-competitive research at a time when few contemporary electric cars existed and wireless power transfer (WPT) systems for EVs were an unproven concept. The SAE J2954 Wireless Power Transfer and Alignment Taskforce worked since 2007 to thoroughly vet and test the technology in partnership with government agencies, regulatory bodies, and private-industry groups including the American Association of Medical Instrumentation (AAMI), U.S. Dept. of Energy (DOE), U.S. Food & Drug Administration (FDA), automotive OEMs, Tier 1 suppliers, and many others.
“Charging your EV should be as simple as parking and walking away — the wireless charging SAE J2954 Standard gives freedom and convenience to do exactly that, safely and automatically,” stated Jesse Schneider, chair of the SAE J2954 Task Force.
WPT systems work by parking in a wireless charging spot with the vehicle positioned over an SAE J2954-compatible ground assembly pad. After a communications handshake, charging begins automatically without a physical corded connection. Power is transferred by creating a magnetic resonance field between the transmitting pad on the ground (wired to the grid) and a receiving pad fitted on the underside of the vehicle. The energy crosses an air gap (the ground clearance between the pads) and is then converted from AC into DC on the vehicle to charge the vehicle batteries.
The technology is a safe and efficient method for transferring power from the AC grid supply to the electric vehicle. Tests using a 10" (250-mm) ground clearance have shown that WPT systems operate at grid-to-battery efficiencies of up to 94%. WPT with additional alignment elements in SAE J2954 also fulfills the charging requirements for autonomous EVs to charge themselves without human interaction.
“The SAE J2954 standard is a game-changer by giving a ‘cookbook’ specification for developing both the vehicle and charging infrastructure wireless power transfer, as one system, compatible to 11 kW,” Schneider said. “The SAE J2954 alignment technology gives additional parking assistance, even allowing for vehicles to park and charge themselves autonomously.”
He noted that the SAE J2954 task force coordinated its efforts with industry and other standards organizations to ensure global harmonization. “Publishing SAE J2954 is a major step forward in wireless charging commercialization for EVs,” Schneider said.
11-kW Universal Ground Assembly
A critical issue addressed early in the SAE J2954 process was to classify products in terms of charging levels, vehicle ground clearance, and systems interoperability. Three power levels were established: WPT1 (3.7 kW), WPT2 (7 kW), and WPT3 (11 kW). The WPT system consists of two “sides.” The Ground Assembly (GA) encompassing the charging hardware is wired into the grid. The other side includes the on-vehicle equipment known as the Vehicle Assembly (VA).
SAE J2954 establishes a universal Ground Assembly for WPT3, critical especially for public infrastructures. It is downward-compatible to charge vehicles also at WPT1 and WPT2. The goal is that the WPT-GAs will be installed in publicly available parking spaces, per the setup in today’s plugin charging infrastructure. Installation with WPT3 will allow downward compatibility.
For ease of use, SAE J2954 specifies the requirements to make the GAs and VAs fully interoperable, so that any vehicle will be able to charge when it is parked in an SAE J2954 GA-equipped parking location. There is also the possibility to have specific designs for captive fleets; as described in SAE J2954 in this case, a GA would only be expected to fully operate with a specific group of vehicles.
To validate its performance targets, safety limits, and methodologies, the SAE J2954 standard includes key parameters such as minimum efficiency, EMI, and EMF (electromagnetic interference and field) limits as well as foreign object detection. There are three overlapping ranges of vehicle ground clearances from 100 to 250 mm (3.9 to 9.8") and three levels of grid input to the GA up to 11.1 kVA. Parking tolerances are ±75 mm (3.0") in the direction of travel and ±100 mm (3.9") in the lateral direction.
The SAE J2954 task force concluded that to ensure interoperability, the ability of systems to transfer power, as designed by different manufacturers, must be validated in both bench and vehicle testing. SAE J2954 standardizes a WPT GA/VA test station along with coil specifications to evaluate the requirements for safety, interoperability and performance. This allowed OEMs and Tier 1s alike to prove their vehicles and charging subsystems were compatible with SAE J2954 requirements and guidelines.
The baseline bench testing of the WPT systems was carried out at both the DOE’s Idaho National Laboratory and TDK RF Solutions, evaluating the GA side and the VA sides. For vehicle systems testing, a group of companies including Aptiv, BMW, Continental, Ford, General Motors, Hevo, Honda, Hyundai, IHI, KAIST, Lear, Qualcomm, Toyota, and WiTricity under SAE leadership created a Cooperative Research Project (CRP) with industry-committed funds for additional vehicle and emissions testing. In those tests conducted at TDK RF Solutions near Austin, TX, automakers and suppliers brought vehicles. Different suppliers brought GAs that were tested for performance, interoperability, and EMI/EMF emissions.