New Charging System for EVs Boosts Efficiency and Integration
A groundbreaking development in electric vehicle (EV) charging technology has emerged from a collaborative research effort led by Guo Lei, a postgraduate researcher at Nanjing University of Aeronautics and Astronautics. The study, recently published in the Transactions of China Electrotechnical Society, introduces a novel dual-battery integrated charging system based on an open-winding permanent magnet synchronous motor (OW-PMSM). This innovative approach not only enhances charging efficiency but also significantly reduces the complexity and cost of onboard charging systems, addressing two of the most pressing challenges in the EV industry today: limited driving range and the high cost of charging infrastructure.
As global demand for electric vehicles continues to rise, so does the need for smarter, more efficient charging solutions. While fast-charging stations have become increasingly common, they come with their own set of limitations—high installation costs, significant land use, and grid strain during peak hours. In contrast, onboard chargers offer a more convenient and scalable alternative, allowing EV owners to charge their vehicles at home or during idle periods. However, traditional onboard chargers are often bulky, heavy, and inefficient, adding unnecessary weight and complexity to the vehicle without contributing to propulsion. This has prompted researchers to explore ways to integrate charging functionality directly into the existing drivetrain, leveraging components that are already present in the vehicle.
The research team, including Wei Jiadan, Wang Yiwei, Zhou Bo, and Wang Yin, has taken this integration concept to the next level by reimagining the role of the electric motor itself. Instead of treating the motor solely as a propulsion device, their system repurposes it during charging mode to function as part of a high-efficiency power conversion circuit. By connecting the three-phase AC grid to the neutral points of the OW-PMSM’s windings, the motor’s phase coils act as filtering inductors, while the dual inverters are reused as rectifiers to convert AC power into DC for battery charging. This eliminates the need for additional magnetic components and power electronics, resulting in a highly compact and lightweight charging solution.
One of the key advantages of using an open-winding motor architecture is its inherent flexibility. Unlike conventional motors with a single set of windings connected in a star or delta configuration, open-winding motors feature two separate three-phase winding sets, each connected to its own inverter. This dual-inverter setup allows for greater control over current flow and enables independent charging of two battery packs—a configuration that is particularly useful in hybrid or dual-voltage EV platforms. In this study, the researchers leveraged this topology to simultaneously charge two high-voltage battery systems, effectively doubling the charging capability without increasing the physical footprint of the system.
However, a major challenge arises when the two battery packs have different states of charge (SOC), which is common in real-world driving conditions. Unequal SOC levels lead to mismatched load impedances during charging, causing unbalanced power distribution between the two channels. This imbalance can result in uneven current flow through the motor windings, generating pulsating electromagnetic torque that may cause mechanical vibrations, noise, and even premature wear on drivetrain components. To address this issue, the team developed a sophisticated control strategy known as quasi-direct power control (QDPC), which actively manages the power flow to maintain balance and suppress torque ripple.
The QDPC strategy operates in the stationary αβ reference frame, bypassing the need for complex coordinate transformations typically required in traditional vector control methods. By directly regulating the instantaneous active and reactive power, the controller ensures that the input current remains sinusoidal and in phase with the grid voltage, achieving near-unity power factor operation. This not only improves charging efficiency but also minimizes harmonic distortion, reducing stress on the power grid. Moreover, the elimination of software-based phase-locked loops (PLLs) and dq-axis decoupling calculations reduces computational load on the digital signal processor, making the system faster and more responsive—especially under dynamic load conditions.
To validate their approach, the researchers conducted extensive simulations and built a full-scale experimental prototype. The test platform featured a 5-pole-pair OW-PMSM with phase inductance values of 8.36 mH (d-axis) and 9.12 mH (q-axis), powered by a 50 Hz, 50 V AC source and controlled using a digital controller with a 40 kHz switching frequency. Two variable resistive loads were used to simulate different battery charging scenarios, with load ratios adjusted to mimic real-world imbalances. Under steady-state conditions, the system demonstrated excellent performance across all test cases. When the load ratio was set to 1.5—indicating a 50% difference in equivalent resistance between the two channels—the total harmonic distortion (THD) of the input current remained below 4.5%, well within acceptable limits for grid-connected equipment. The average charging torque was kept under 1.2 N·m, with peak-to-peak ripple amplitudes never exceeding 1.26 N·m, even under the most extreme imbalance conditions.
In dynamic tests, the system’s responsiveness was further highlighted. During variable-load experiments where the load ratio shifted from 1.5 to 0.75 and back, the QDPC controller successfully maintained stable output voltages and minimized power fluctuations. The maximum deviation in average active power was only 18.43 W, and the peak torque ripple remained below 1.21 N·m throughout the transition. In comparison, a conventional power balance control strategy exhibited significantly higher disturbances, with power deviations reaching 32.75 W and torque ripple peaking at 1.76 N·m—nearly 50% worse than the QDPC method. These results underscore the superiority of the proposed control algorithm in maintaining system stability and minimizing mechanical stress during transient events.
Voltage variation tests were also conducted to simulate the changing terminal voltage of a battery during the charging cycle. As the reference voltage was ramped up from 168 V to 200 V, the system smoothly adjusted the input current to meet the increased power demand, with no abrupt spikes or oscillations in torque. The input current rose from approximately 3.8 A to 10 A during the transition and settled back to around 5.2 A once the new voltage level was reached. Throughout this process, the charging torque followed the expected trend, increasing slightly with current magnitude but remaining tightly regulated. When the voltage was reduced back to 168 V, the system responded equally well, demonstrating symmetrical behavior and confirming its robustness under bidirectional transients.
One of the most compelling aspects of this technology is its potential for widespread adoption in next-generation EVs. Unlike many advanced charging concepts that require exotic materials or specialized hardware, this solution builds upon existing motor and inverter architectures already found in many electric vehicles. The open-winding motor, though not yet mainstream, is gaining traction in high-performance and commercial applications due to its superior fault tolerance, thermal management, and power density. By integrating charging functionality into such a motor, automakers can reduce component count, lower manufacturing costs, and improve overall system reliability.
Furthermore, the ability to charge two battery packs independently opens up new possibilities for vehicle design. For example, one battery could be optimized for high energy density (long-range driving), while the other prioritizes high power density (rapid acceleration and regenerative braking). During normal operation, both batteries supply power to the motor, but during charging, they can be replenished at different rates depending on their individual SOC and chemistry. This level of granularity is difficult to achieve with conventional single-battery systems and could lead to longer battery life and improved thermal management.
From a sustainability perspective, the reduction in material usage and electronic waste is another significant benefit. Traditional onboard chargers rely on bulky inductors and capacitors made from rare earth metals and other environmentally sensitive materials. By reusing the motor windings as filtering elements, this new system eliminates the need for these components, contributing to a greener lifecycle for EVs. Additionally, the improved power factor and reduced harmonic emissions mean that the charger places less strain on the electrical grid, supporting the broader goal of clean energy integration.
The implications of this research extend beyond passenger vehicles. Commercial fleets, delivery vans, and urban buses—many of which operate on fixed routes and return to depots each night—could greatly benefit from such an integrated charging solution. The ability to charge quickly and efficiently using standard AC power, without requiring expensive DC fast-charging infrastructure, makes this technology particularly attractive for fleet operators looking to minimize downtime and operational costs.
Despite its many advantages, the system is not without limitations. The assumption of a perfectly balanced motor winding set, for instance, may not hold true in mass production, where manufacturing tolerances can introduce slight asymmetries. Similarly, the effectiveness of torque cancellation depends on precise current control, which may be affected by sensor inaccuracies or parameter drift over time. Future work will likely focus on adaptive control techniques that can compensate for these non-idealities in real time, further enhancing system robustness.
Another area for exploration is the extension of this concept to other motor types, such as switched reluctance motors or induction machines, which may offer different trade-offs in terms of cost, efficiency, and controllability. Additionally, integrating vehicle-to-grid (V2G) capabilities into the same architecture could enable bidirectional energy flow, allowing EVs to not only draw power from the grid but also feed it back during peak demand periods—a key enabler of smart grid technologies.
In conclusion, the dual-battery integrated charging system developed by Guo Lei and his colleagues represents a significant step forward in the evolution of EV power electronics. By seamlessly blending propulsion and charging functions within a single motor-inverter unit, the system achieves a level of integration and efficiency that was previously unattainable. The quasi-direct power control strategy further enhances performance by minimizing torque ripple and ensuring stable operation under unbalanced loading conditions. With successful validation through both simulation and experimentation, this technology is poised to play a pivotal role in the next generation of electric vehicles, helping to accelerate the transition to a cleaner, more sustainable transportation future.
Guo Lei, Wei Jiadan, Wang Yiwei, Zhou Bo, Wang Yin, Transactions of China Electrotechnical Society, DOI: 10.19595/j.cnki.1000-6753.tces.230426