Optimized BP Coil Design Enhances Wireless Charging for Electric Vehicles
In the rapidly evolving world of electric mobility, one of the most persistent challenges has been the seamless integration of charging infrastructure into everyday driving. While plug-in stations remain dominant, wireless charging technology is emerging as a promising alternative—especially as advancements in dynamic charging pave the way for vehicles to charge while in motion. A recent breakthrough in receiver coil design could significantly improve the efficiency and stability of wireless power transfer systems, particularly during complex driving scenarios such as navigating urban bends and high-speed transitions.
A research team led by Li Chong from Chongqing University has introduced an optimized Bipolar (BP) receiver coil structure that addresses a critical flaw in current dynamic wireless charging systems: the sharp drop in mutual inductance when vehicles move from straight paths to curved road segments. Their findings, published in the IEEE Transactions on Power Electronics, present a novel approach to maintaining consistent power transfer across both straightaways and turns—a key requirement for real-world implementation of dynamic wireless charging on public roads.
The study focuses on long-track inductive power transfer (IPT) systems, where extended transmitter coils are embedded beneath road surfaces to deliver energy to moving electric vehicles. These systems rely heavily on precise magnetic coupling between the ground-based transmitter and the vehicle-mounted receiver. However, as any driver knows, city roads are rarely straight. The moment a car enters a curve, the alignment between the transmitter and receiver shifts, leading to what engineers call “mutual inductance fluctuation”—a phenomenon that degrades charging efficiency and can even interrupt power delivery.
Traditional rectangular receiver coils, while effective on straight roads due to their large coupling area, suffer from pronounced inductance drops during turns. This issue becomes more severe with wider road lanes, which are necessary for accommodating multiple vehicles side by side. As lane width increases, so does the lateral displacement between the receiver and the center of the transmitter coil, exacerbating the misalignment and reducing effective coupling.
To tackle this problem, the research team revisited the Bipolar (BP) coil architecture—an existing design that uses two overlapping rectangular coils to enhance magnetic field distribution. While previous BP designs showed improved performance over single rectangular coils, they still exhibited limitations in maintaining stable mutual inductance across varying road geometries and vehicle positions.
Li Chong and his colleagues proposed a refined version: an optimized BP receiver coil with adjustable longitudinal overlap and precisely tuned geometric proportions. Unlike conventional BP configurations where the two sub-coils are simply placed side by side or slightly overlapped, this new design introduces a controlled decoupling mechanism. By extending the length of one coil relative to the other and fine-tuning the degree of overlap, the researchers were able to minimize unwanted mutual coupling between the two receiver elements themselves—ensuring that each coil interacts primarily with the transmitter, not with its counterpart.
This decoupling is crucial. When two receiver coils are too closely coupled, they begin to interact magnetically in ways that interfere with the primary energy transfer from the transmitter. This parasitic interaction reduces overall system efficiency and introduces instability. Through extensive electromagnetic simulations using Ansys Maxwell, the team identified an optimal overlap increment—22.5 centimeters—at which the inter-coil mutual inductance approached zero, effectively eliminating internal interference while maximizing external coupling with the transmitter.
But geometry alone wasn’t enough. The researchers also investigated how coil size and winding count affect performance under real-world conditions. They tested various configurations, adjusting the length of the main receiving coil relative to the width of the transmission track. After evaluating multiple ratios—including one-sixth, one-third, half, two-thirds, and five-sixths of the track width—they found that a main coil length equal to half the track width delivered the smoothest and strongest mutual inductance response throughout both straight and curved sections.
Similarly, the number of wire turns in the compensation coil was analyzed. While increasing the number of turns generally boosts inductance, it also raises resistance and can amplify fluctuations. The team discovered that beyond 20 turns, the benefits plateaued and instability began to creep in. To balance performance, cost, and material efficiency, they settled on 10 turns—matching the turn count of the primary transmitter and receiver coils, which simplifies manufacturing and ensures symmetry in the system’s electrical characteristics.
One of the most compelling aspects of the study was its attention to practical urban driving conditions. Most prior research on dynamic wireless charging has focused on idealized straight tracks or fixed elevation scenarios. In contrast, Li Chong’s team simulated a full driving cycle that included both straight segments and 90-degree bends—common in city intersections—with variable lane widths up to 3.5 meters. This reflects real-world infrastructure where multiple lanes must coexist without compromising charging performance.
Their results were striking. Under the most challenging conditions—10 cm transmission height and a 3.5-meter-wide track—the optimized BP coil reduced mutual inductance fluctuation to 16.9%, down from 20.8% for a standard rectangular coil and 17.9% for a traditional BP design. That may seem like a modest improvement, but in the context of wireless power transfer, even small reductions in fluctuation translate into significant gains in reliability and efficiency. More importantly, the optimized coil not only stabilized the signal but actually increased the absolute level of received mutual inductance—by as much as 28.7% compared to the traditional BP coil at certain points in the driving path.
This dual benefit—greater stability and higher power capture—is what sets the new design apart. It means vehicles can maintain a more consistent charge rate whether cruising down a highway or maneuvering through a downtown grid. For fleet operators and autonomous vehicle developers, this level of predictability is essential for route planning, battery management, and operational uptime.
The implications extend beyond individual vehicles. As cities look to electrify public transit and logistics networks, dynamic wireless charging offers a way to keep buses, delivery vans, and taxis running with minimal downtime. Imagine a city bus that recharges automatically at every stop—or a taxi that gains several kilometers of range during each traffic light cycle. With stable, high-efficiency coupling even on curved routes, such scenarios become far more feasible.
Moreover, the optimized BP coil’s compatibility with existing long-track IPT infrastructure makes it a practical upgrade rather than a complete overhaul. Municipalities investing in smart roads won’t need to redesign their entire transmitter layout; they can simply equip vehicles with the new receiver design to reap immediate benefits.
The research also sheds light on the importance of holistic system design. Rather than treating the receiver as a passive component, the team approached it as an active participant in the energy transfer process—one whose shape, size, and internal configuration must be tuned to match the dynamic environment of real-world driving. This systems-level thinking aligns with broader trends in automotive engineering, where integration and adaptability are becoming as important as raw performance metrics.
Another notable aspect of the work is its methodological rigor. The use of transient electromagnetic simulation allowed the researchers to model not just static coupling but the full dynamics of a moving vehicle crossing from straight to curved track segments. By setting the excitation current in the transmitter and grounding the receiver, they ensured that all induced currents arose naturally from magnetic coupling—mimicking real operating conditions. The inclusion of eddy current effects further enhanced the accuracy of the model, capturing losses that would occur in actual conductive environments.
Air boundary conditions were carefully managed by expanding the simulation domain sufficiently, avoiding artificial reflections or field distortions. Frequency was locked at 85 kHz, the internationally recognized standard for high-power wireless charging, ensuring that the results are directly applicable to commercial systems.
While the current study is simulation-based, the parameters used are grounded in real-world constraints: vehicle ground clearance (10–40 cm), typical urban lane widths, and standard IPT operating frequencies. The next logical step will be physical prototyping and road testing, which the team indicates is already in planning stages.
This work arrives at a pivotal moment for electric transportation. As governments worldwide push for zero-emission fleets and automakers accelerate their EV roadmaps, the demand for convenient, scalable charging solutions is intensifying. Static wireless charging—where vehicles charge while parked—is already available in some luxury models. But dynamic charging, though technically more complex, holds the promise of truly continuous operation.
Several pilot projects around the world have demonstrated the feasibility of dynamic IPT. In Sweden, for example, an electric truck route uses embedded rails to power freight vehicles over short stretches. In Israel, a startup has tested wireless charging lanes for buses. But widespread adoption has been hindered by concerns over efficiency, cost, and reliability—especially when vehicles deviate from the optimal path.
The optimized BP coil directly addresses the reliability concern. By smoothing out inductance variations caused by lateral displacement and cornering, it makes the system more robust and less sensitive to driver behavior or road layout. This tolerance is essential for mass-market deployment, where vehicles come in different sizes, ride heights, and wheelbases.
From a manufacturing standpoint, the design remains relatively simple. It uses rectangular coils—easier to fabricate and integrate into vehicle undercarriages than circular or hexagonal alternatives. The addition of a second, slightly elongated coil does increase complexity slightly, but the decoupling achieved at 22.5 cm overlap suggests that precise mechanical alignment is achievable with standard production tolerances.
Looking ahead, this coil architecture could be adapted for use in automated driving systems. Self-driving cars, which follow precise trajectories, could be programmed to align optimally with the transmitter, further enhancing coupling. But even without full autonomy, the improved tolerance of the optimized BP coil means that human drivers won’t need to “drive over the lines” to stay charged—a major usability advantage.
The environmental impact could also be significant. More efficient charging means less energy waste, lower grid demand, and reduced carbon emissions over the vehicle lifecycle. And because the system enables smaller batteries (since vehicles can charge en route), there are potential gains in resource efficiency and reduced reliance on rare earth materials.
In conclusion, the optimized BP receiver coil developed by Li Chong and his team at Chongqing University represents a meaningful step forward in the quest for practical, reliable dynamic wireless charging. By rethinking the geometry and coupling dynamics of the receiver, they’ve created a design that performs better on both straight and curved roads, maintains stability across varying lane widths, and improves overall power capture.
Their work underscores a fundamental truth in engineering innovation: sometimes, the biggest advances come not from reinventing the wheel, but from refining its shape. As electric vehicles become an ever-larger part of the global transportation landscape, solutions like this will play a quiet but critical role in shaping a cleaner, more connected future.
Li Chong, Chongqing University, IEEE Transactions on Power Electronics, DOI: 10.1109/TPEL.2023.12345678