New Wireless Charging System Offers Stable Power for EVs
A groundbreaking advancement in electric vehicle (EV) wireless charging has emerged from a collaborative research effort between Hunan University of Technology and Hunan University. Led by Dr. Li Zhongqi, along with colleagues Zhang Chenxi, Wang Jianbin, Wang Zhongmei, and Professor Huang Shoudao, the team has developed an innovative wireless power transfer (WPT) system designed to deliver stable constant current and constant voltage output, even under challenging real-world conditions such as coil misalignment and variable loads. Published in the prestigious Transactions of China Electrotechnical Society, this work addresses one of the most persistent hurdles in making wireless EV charging as reliable and user-friendly as conventional plug-in methods.
The core challenge in wireless charging lies in maintaining consistent power delivery. Unlike wired systems where physical connectors ensure a direct and stable link, WPT relies on electromagnetic fields between transmitter and receiver coils. The efficiency and stability of this energy transfer are highly sensitive to the distance and alignment—known as the coupling coefficient—between these coils. In practical scenarios, such as parking an EV over a ground-mounted charging pad, perfect alignment is nearly impossible. Even minor shifts can drastically alter the magnetic coupling, leading to fluctuations in output current and voltage. This instability is particularly problematic for lithium-ion batteries, which require precise charging protocols: an initial phase of constant current (CC) followed by a constant voltage (CV) phase to maximize battery life and safety. A system that cannot maintain these profiles risks inefficient charging, reduced battery lifespan, or even potential damage.
Existing solutions have often been complex and costly. Many prior approaches involve intricate compensation networks with numerous capacitors and inductors, or the use of multiple switching components on either the transmitter or receiver side to reconfigure the circuit topology for different charging modes. While effective in theory, these designs increase system complexity, cost, component count, and potential points of failure. Furthermore, some proposed systems operate at frequencies outside the internationally recognized standards, such as the SAE J2954 band of 79–90 kHz, which is crucial for ensuring interoperability and minimizing electromagnetic interference.
The new system introduced by Li and his team elegantly sidesteps these issues with a dual-pronged strategy centered on two key innovations: a novel “staggered stacked coil” (SSC) structure and a “variable frequency reconfiguration S/SP” control strategy. This combination allows the system to achieve both high immunity to positional errors and seamless switching between CC and CV modes with remarkable simplicity.
The first pillar of their innovation is the SSC coil design. Traditional rectangular coils suffer from a significant drop in coupling when the receiver is offset from the center of the transmitter. The researchers tackled this by enhancing a standard rectangular main coil with additional smaller “compensation coils” placed strategically at its edges. On the transmitter side, two compensation coils are stacked parallel to each other along the Y-axis on top of the main coil. On the receiver side, two are stacked along the X-axis. This asymmetrical but deliberate arrangement creates a more robust and uniform magnetic field distribution.
The physics behind this design is rooted in the principle of magnetic field superposition. When the receiver is perfectly aligned, the main coils provide the primary coupling. As the vehicle begins to move off-center—say, shifting along the X-axis—the portion of the main receiving coil that remains over the strong central field of the transmitter starts to see reduced flux. However, at the same time, one of the edge compensation coils on the receiver moves into a region of stronger field, while the other moves out. The key insight is that the rate at which the coupling increases in the newly engaged compensation coil can be engineered to counterbalance the rate at which it decreases in the main coil and the other compensation coil. This results in a total mutual inductance—and thus a total coupling coefficient—that remains remarkably stable over a wide range of offsets. It’s not about eliminating variation, but about creating a controlled response that flattens the overall performance curve across a broad operating zone.
To optimize this effect, the research team employed a sophisticated computational approach. They treated the coupling coefficient as a target function and used an algorithm to systematically vary parameters like the inner dimensions and number of turns in both the main and compensation coils. The optimization goal was to minimize the fluctuation rate of the coupling coefficient within defined offset ranges. After extensive simulation and analysis, they arrived at a set of optimal parameters that would form the basis of their experimental prototype.
The second major innovation is the variable frequency reconfiguration strategy. Instead of adding extra switches or complex circuitry, the team devised a method that uses a single mechanical switch on the receiver side and a simple change in operating frequency to toggle between charging modes. The system operates with two distinct compensation topologies: S/S for constant current and S/SP for constant voltage. In the S/S configuration, only series capacitors are used on both the transmitter and receiver sides. In the S/SP configuration, an additional capacitor is switched into a parallel connection on the receiver side via a single switch.
The brilliance of the design lies in how the compensation components are shared. The values of the capacitors are carefully calculated so that the same physical components can serve both topologies. By adjusting the operating frequencies of the two modes according to a specific mathematical relationship derived from the system’s electrical characteristics, the capacitors required for resonance in the CC mode are identical to those needed in the CV mode. This eliminates the need for duplicate or redundant components, drastically reducing the system’s complexity and size.
When the vehicle begins charging, the system operates in S/S mode at a frequency of 80.5 kHz. At this point, the single switch is open, and the circuit is configured for constant current output, ideal for the initial bulk-charging phase. Once the battery reaches a certain voltage threshold, the control system closes the switch, connecting the parallel capacitor, and simultaneously changes the operating frequency to 85.0 kHz. This seamlessly transitions the system into S/SP mode, providing the constant voltage output needed for the final charging stage. Both of these frequencies comfortably sit within the SAE J2954 standard band, ensuring compatibility and regulatory compliance.
The entire system is also designed to operate at zero phase angle (ZPA), meaning the input voltage and current are perfectly in phase. This condition maximizes power transfer efficiency and minimizes reactive power, allowing the inverter to operate under the most favorable conditions. Achieving ZPA in both modes without additional tuning is another testament to the elegance of the design.
To validate their theoretical work, the team constructed a 500-watt experimental prototype. The physical coils were meticulously wound according to the optimized parameters, and the system was tested on a precision motion platform to simulate various degrees of misalignment. The results were impressive. When the receiver was shifted along the X-axis by up to 187 millimeters—a full 55% of the transmitter’s outer diameter—the coupling coefficient fluctuated by less than 5%. For context, many conventional systems see their coupling drop by 30-50% or more with similar offsets. Similarly, a 120-millimeter shift along the Y-axis resulted in a coupling fluctuation of just 1%, demonstrating excellent isotropic performance.
More importantly, the system delivered on its promise of stable output. In constant current mode, the output current remained fixed at 6.3 amperes with a fluctuation rate of only 3.58%, well within acceptable limits for battery charging. In constant voltage mode, the output held steady at 74 volts with a maximum fluctuation of 4.83%. These low ripple values confirm that the system effectively isolates the load from the disturbances caused by coil misalignment and load variations. Efficiency measurements showed peak transmission efficiencies of 91.24% in CC mode and 93.3% in CV mode, with performance remaining high even at significant offsets, further proving the robustness of the SSC design.
The practical implications of this research are substantial. For automotive manufacturers, this technology offers a path to simpler, lighter, and more cost-effective wireless charging systems. The reduction in component count—from seven or more capacitors in some prior art to just three in this design—directly translates to lower production costs, less space required in the vehicle’s undercarriage, and improved reliability. For consumers, it means a truly convenient charging experience. Drivers will no longer need to park with surgical precision; a generous margin of error will still result in a fast, efficient, and safe charge. This ease of use is critical for widespread adoption.
From a broader industry perspective, this work represents a significant step toward standardized and interoperable wireless charging infrastructure. By adhering to the SAE J2954 frequency band and demonstrating high performance with a simple topology, this system could become a benchmark for future development. It shows that high performance does not necessitate high complexity, a valuable lesson for engineers striving to bring cutting-edge technology to the mass market.
The success of this project also highlights the importance of integrated, multidisciplinary research. The solution did not come from focusing solely on electronics or solely on magnetics. Instead, it arose from a deep understanding of both domains and how they interact. The coil structure was not an afterthought but a co-designed element that enabled the simplified electronic control strategy. This holistic approach is increasingly essential in solving complex engineering challenges in the modern world.
While the current prototype is a 500-watt system, the principles are scalable. The same fundamental concepts of staggered stacked coils and variable frequency reconfiguration could be applied to higher-power systems required for larger vehicles or faster charging. The research team has already identified the next steps, including the development of a fully automated control system that can sense the battery’s state of charge and seamlessly transition between CC and CV modes without manual intervention.
In conclusion, the work of Li Zhongqi, Zhang Chenxi, Wang Jianbin, Wang Zhongmei, and Huang Shoudao presents a compelling vision for the future of EV charging. By ingeniously combining a mechanically robust coil design with an electronically elegant control strategy, they have created a system that is not only technically superior but also eminently practical. It stands as a powerful example of how innovative thinking can overcome longstanding technical barriers, bringing us closer to a world where refueling an electric car is as effortless as parking it.
Li Zhongqi, Zhang Chenxi, Wang Jianbin, Wang Zhongmei, Huang Shoudao, Transactions of China Electrotechnical Society, DOI: 10.19595/j.cnki.1000-6753.tces.232107