The Unique Safety Concerns of Wireless Charging
Joe Bablo, Ken Boyce, and Hai Jiang
Underwriters Laboratories (UL)
The development of wireless charging systems for electric vehicles has been in process for some time. This technology offers the user a “hands free” method of charging, but also brings along some safety aspects that are unique to wireless charging.
Standardization efforts around design and safety are in process, and these standards will be needed to insure safe equipment that is functional and able to operate as intended when used in various combinations of charge station and vehicle (interoperability).
This paper will focus on the unique aspects of wireless charging as they relate to safety and potential safety certification of the off board Electric Vehicle Supply Equipment (EVSE), how these issues are different when compare to conductive charging, and how those differences affect the way certification can be handled.
Description / Examples
Before we begin our discussion, it would be a good idea to define what it is we are talking about. Wireless charging has distinct use scenarios, which parallel conductive charging in some cases – Residential Charging, Commercial Charging, High-Power Charging – and are completely unique to wireless charging in some others – Dynamic Charging. Each of these will be discussed below.
Residential Charging – Installing a wireless charging system at the user’s residence allows the user to charge their vehicle wirelessly, but also allows the system to interact with a specific vehicle. This “mated pair” aspect allows for insured interoperability of the charge station and the vehicle, because a vehicle owner would not install a charge station that would not work properly with their vehicle. In this type of scenario, some safety aspects are controlled due to the “mated pair” aspect as well.
Commercial Charging – Installation of a wireless charging system in a public location allows any vehicle equipped with wireless charging technology to utilize the charging spot. This use is different than residential use in that the “mating” that occurs between the vehicle and the charge station is uncontrolled in the sense that any combination of charge station and vehicle is possible. This variable use aspect brings interoperability concerns to the forefront. There is not only a concern that the vehicle will charge when it is supposed to, but that the vehicle and charging system safety systems will work properly together when used in the real world.
Dynamic Charging – This type of charging is unique to wireless charging. Charging occurs as the vehicle moves over a prescribed path. Installed in the road surface along this path are charge coils that will couple with the vehicle and charge the vehicle as it passes over the coil. With this method, a vehicle can receive a charge current continuously as it drives down a dynamic charging road. In a sense, this has the same interoperability concerns as commercial charging, but there are also added safety concerns due to the movement of the vehicle.
High-Power (Bus) Charging – In order to charge larger commercial type vehicles or to potentially charge light duty passenger vehicles quickly, similar to fast charging in conductive charging terminology, there is some work being done in the area of high-power charging. This type of charging is basically the same as normal wireless charging but at much higher power levels that involves an increased level of concern.
These terms and concepts will be used to define the standards development activities and the safety aspects discussion that follows.
All of the concern with interoperability and the control of safety aspects of these systems can and will be addressed by the standards that are being developed around this technology. The standards under development are looking at residential and commercial charging, as well as high powered charging, and addressing the issues through design and safety requirements. For dynamic charging, there is some work being done, but this technology is considered to be a little further down the road when compared to residential and commercial charging, and is therefore not currently a primary focus for standardization activities.
Standardization is underway for wireless charging. The Society of Automotive Engineers (SAE) and UL are currently collaborating in this area. This includes drafting an SAE guideline document for wireless system with the intent of addressing and focusing on design, efficiency, vehicle safety systems, and interoperability. Interoperability is a concept that all charging stations will work with all vehicles, including not just the start of charging, but the safety and proper functioning of the system. Vehicle safety systems, used to mitigate hazards, will outline what is needed on the vehicle side to address specific safety concerns.
Underwriters Laboratories is creating a standard addressing the safety of the wireless charging system, addressing fire, shock, and injury to persons in relation to the off board power source and coils. Additionally, safety issues that are not covered by vehicle systems will fall to the UL document to provide requirements for mitigating the hazard.
These two documents are being developed in conjunction with one another to control gaps and overlaps such that the standard will be able to work together as this technology matures. In the event that a wireless charging system will not comply with the SAE document for design, further requirements will be necessary to address the vehicle system.
Unique Safety Concerns
There are normal safety concerns that apply to almost all electrical products. This includes risk of fire and risk of shock. These safety concerns are addressed in similar, if not identical, ways between different product types. In wireless charging systems, these same safety concerns exist for the off board EVSE. However, for wireless charging there are also some very unique safety concerns that are only related to this type of charging. These unique concerns are detailed below.
Vehicle Systems – When a vehicle is charging, the vehicle itself becomes part of the charging circuit. For conductive charging, the vehicle is capable of being monitored through that conductive connection. This monitoring allows the off board EVSE to react in the case that a hazardous condition is identified, whether that is ground current that exceeds limits, or loss of isolation to the vehicle frame, etc. In the case of wireless charging, the ability of the off board EVSE to monitor the vehicle is lost. All monitoring in now part of the vehicle system, and when hazards are detected, wireless communication must be used to tell the EVSE to shut down. This wireless communication requires functional safety evaluations to insure that the off board EVSE will react correctly when messages are received. However, the actual detection means and the reliability of the communication are vehicle issues, and unless those vehicle systems are included in the evaluation of the equipment, reliance on these systems for safety cannot be included in the certification of the off board EVSE.
Field Exposure – Wireless charging is accomplished through a coupling of the primary and secondary coils. This involves the generation of electrical and magnetic fields that can be potentially dangerous to the user or others in the vicinity of the vehicle while charging. Although design of the system and the vehicle are used to protect the user from exposure to these fields, the hazard of exposure still needs to be addressed through the evaluation process of the wireless charging system. This is addressed by specific zones being outlined and exposure limits assigned to those zones. However, if the user was to attempt to enter a zone that was not intended, such as reaching for dropped keys that are partially under the vehicle, then a system is needed to protect the user. That system requires the use of physical and/or tangible means to restrict access or the system needs to be able to shut down to prevent exposure under those conditions. If the means to protect the user is part of the off board EVSE system, or part of a vehicle system that is included in the evaluation process, then the reliability and applicability of the system can be assessed. If the system is vehicle based and not included in the evaluation process, then its reliability cannot be judged as part of the certification process.
Object Detection – One type of protection system that is intended to be included is object detection. This applies to both organic object detection and non-organic object detection. These detection systems are intended to detect an object entering an area that is normally restricted, and then signaling the power source to shut down and remove power so that the hazard is avoided. If these systems are part of the off board EVSE, or if they are part of a vehicle system that is included in the evaluation process, then the reliability and applicability of the system can be reassessed. If the system in vehicle based and not included in the evaluation process, then its reliability cannot be judged as part of the certification process for the off board EVSE.
Misalignment – Misalignment of the coils will affect the coupling factor of the system. As such, this will affect the efficiency of the power transfer. However, there are also concerns that misalignment can distort the field in a way that would allow the field to stretch or extend into an adjacent zone. For example, the user cannot be located under the vehicle during charging, but can be located in next to or within the vehicle. If misalignment can cause the field to extend from under the vehicle into the user occupied area next to the vehicle, then a hazardous condition could exist. If this is the case, then misalignment of the coils would become a safety critical issue. Maximum misalignment would need to be controlled for more than just efficiency, and the means to control or mitigate misalignment would become a safety critical system.
Wireless charging has some unique aspects that need to be addressed by standards in order to be able to insure that these systems are safe for use. As mentioned, the work being done in standards development by UL and SAE, will address these particular safety concerns. As the technology matures, the variance in design is expected to decrease in order to drive consistency in addressing the safety concerns. Overall, the work completed so far has taken great strides in addressing the safety concerns with this technology.
Joe Bablo is a Primary Designated Engineer (PDE) with Underwriters Laboratories (UL). Joe is the PDE for Automotive Equipment and Associated Technologies, which spans products from gasoline dispensers to Electric Vehicle Charging Equipment. Joe has been with UL 19 years, the last 11 in his current role. In addition, Joe serves as a committee member for the Society of Automotive Engineers (SAE) as well as a committee member in IEC committees for conductive charging and wireless charging of electric vehicles. Joe also serves as an alternate to Code Making Panel 12 as a technical expert for Article 625 of the National Electrical Code. Joe holds a Bachelor of Science in Electrical Engineering from the Milwaukee School of Engineering.