New SAE Wireless Charging Standard Is EV Game-Changer
SAE J2954 paves the way for electric vehicle charging without a plug and enables alignment for manual/autonomous parking.
SAE International on October 22 announced publication of the first global standard that specifies, in a single document, both the electric vehicle- and supply equipment (EVSE) ground-system requirements for wireless charging of electric vehicles (EV). 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), the 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-in. (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 ‘cook-book’ 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 which 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 plug-in 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 in.) and three levels of grid input to the GA up to 11.1 kVA. Parking tolerances are ±75 mm (3.0 in.) in the direction of travel and ±100 mm (3.9 in.) 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 sub-systems 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 Laboratories and TDK RF Solutions, evaluating the GA side and the VA sides. Since these were prototype systems, the SAE J2954 defined VA/GA test station was built to allow for consistent tests conditions over the range of parking alignments, output voltages and ground clearances.
First paired GAs and VAs provided by the same manufacturer – a so-called “matched subsystem” was tested. Performance data (input power, power factor, output power, efficiency, maximum power presented to the load) at various output voltages, along with misalignment conditions and vehicle ground clearances was evaluated. The results of the bench testing showed that the performance targets were realistic and different suppliers, following SAE J2954, could meet requirements.
The second phase of testing was performed with GAs and VAs from different manufacturers to establish non-matched interoperable configurations. The goal was to demonstrate that these different system sub-components could work together; even though there had been no prior verification. The results showed that with competitor interoperable configurations, performance could be reached similar to matched systems, hence proving interoperability between competitor GA/VA following SAE J2954. The work was published under “Bench Testing Validation of Wireless Power Transfer up to 7.7kW Based on SAE J2954 ,” SAE Int. J. Passeng. Cars - Electron. Electr. Syst. 11(2):89-108, 2018.
Safety and emissions testing
With the initial bench performance test results, the coil specifications for the GA and VA – the factors to enable interoperable power transfer – were validated. Further testing was done related to safety and emission aspects, as well as the transition from test-bench measurements including real vehicle measurements.
A final test-bench study was done to evaluate the impact of WPT on Cardiac Implantable Electronic Devices (CIED) was considered in consultation with the AAMI and ISO. WPT systems of different coil topologies were brought to the FDA laboratory for evaluation of the impact on actual medical devices (running pacemakers) during WPT. The results of the FDA tests resulted in agreements on limits and system requirements and solidified that the public infrastructure GA would be circular topology, proven to be within SAE J2954 and AAMI limits.
Thereafter, vehicle systems were to be tested. 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, Texas, automakers and suppliers brought vehicles. Different suppliers brought GAs that were tested for performance, interoperability, and EMI/EMF emissions.
The focus of those tests was to determine the field emissions using calibrated and certified equipment and industry standard procedures so that equipment developers could determine areas of improvement and to correlate with their own laboratory test results. These tests were done in both matched and interoperable configurations, verifying that the integration of the VAs onto the vehicles and the results in final validation of SAE J2954. The work was published under “Validation of Wireless Power Transfer up to 11kW Based on SAE J2954 with Bench and Vehicle Testing ,” SAE Technical Paper 2019-01-0868, 2019.
SAE spearheaded harmonization with numerous Standards Development Organizations (SDOs) and regulatory agencies including FCC, FDA, ITU-R and CEPT. To help harmonize worldwide standardization and ensure as much compatibility as practical across these documents, SAE established an MOU with ISO and Underwriters Laboratories for their UL 2750 Document for EVSE Certification.
In 2019, the SAE J2954 team hosted a joint meeting with leadership from ISO, IEC, and the GB/T standards groups to ensure international compatibility of standards. Incompatibilities between the documents in development were identified and actions were given to each of the organizations to modify their documents to minimize differences. As a result, a homologation agreement was made so that systems compliant with these standards will work around the globe.
In 2020, ISO 19363 created a vehicle-side standard (VA only), and IEC/ISO continue in development of standards with its four-document series (IEC-61980-1, -2 and -3, plus ISO 15118-8 and ISO 15118-20). China published part of the GB/T 38775-series and continues to work on its eight documents. SAE J2954, published in 2020, also enables worldwide harmonization of WPT. Looking to the future, the SAE J2954 Taskforce has begun a new effort for higher power WPT with SAE J2954/2 also for busses and heavy-duty vehicles.
Wireless communication for control of the WPT charging process had been proprietary for each supplier until SAE J2847/6 was published in 2015. In 2020, the SAE Hybrid/ EV communications taskforce updated the J2847/6 document by leveraging the work of the SAE J2954 Alignment and Controls Sub-Team extended a JSON-based message set (protocol) originally developed to bench test wireless energy transfer interoperability between unmatched GA and VA systems.
“SAE J2847/6 is a communications document utilizing WiFi, IEEE 802.11n, designed specifically with the SAE J2954 standard and facilitates the automatic wireless charging experience while allowing for continuous optimization of the WPT system.” stated Ky Sealy, co-lead of SAE J2847/6. This is described in SAE J2847/6 – Communication for Wireless Power Transfer Between Light-Duty Plug-in Electric Vehicles and Wireless EV Charging Stations.