Double-Layer Ring Equalizer Boosts EV Battery Efficiency
In the rapidly evolving world of electric vehicles (EVs), battery performance and longevity remain at the forefront of innovation. A new breakthrough in battery management systems is poised to redefine how energy is balanced within lithium-ion battery packs. Researchers from Yancheng Institute of Technology have developed a novel double-layer ring equalizer that significantly enhances both the speed and efficiency of battery cell balancing, addressing long-standing limitations in traditional architectures.
The team, led by Han Xinsheng, a master’s candidate, and Professor Kan Jiarong, has introduced a hybrid topology combining Buck-Boost converters and switched-capacitor circuits in a dual-tier ring configuration. This innovative design was published in the April 2024 issue of Electronic Science and Technology, a peer-reviewed journal hosted by Xidian University. The study presents a compelling advancement in active battery equalization, offering a practical solution to the persistent challenges of energy loss and slow response times in large battery strings.
Lithium-ion batteries power the modern EV revolution, prized for their high energy density, long cycle life, and stable charge retention. However, when individual cells are connected in series to achieve higher voltages, inherent variations in capacity, aging rates, and thermal distribution can lead to voltage imbalances. Without proper management, these discrepancies result in reduced usable capacity, shortened battery lifespan, and increased safety risks such as overcharging or thermal runaway.
To mitigate these issues, battery management systems (BMS) employ equalization techniques. Passive methods dissipate excess energy from higher-voltage cells through resistors, a simple but inefficient process that wastes energy as heat. Active equalization, on the other hand, transfers energy from stronger to weaker cells, preserving overall energy and improving system efficiency. Among active topologies, the Buck-Boost converter has gained popularity due to its simplicity and effectiveness. However, as the number of cells increases, the energy transfer path becomes longer, leading to higher losses and reduced efficiency.
Existing ring-type equalizers, while offering bidirectional energy flow, still suffer from extended transfer routes and suboptimal performance when scaling to larger packs. The research team recognized that a structural redesign was necessary to overcome these limitations. Their solution—a double-layer ring architecture—introduces a hierarchical approach to energy redistribution.
The proposed system organizes a 12-cell battery pack into three modules of four cells each. Within each module, adjacent cells are connected via Buck-Boost circuits, while the first and last cells in the module are linked through switched-capacitor circuits. This internal arrangement forms the “bottom layer” ring, enabling fast and efficient balancing within the module. At the top level, the modules themselves are interconnected in a similar ring structure—Buck-Boost links between adjacent modules and switched-capacitor links between the first and last modules—forming the “top layer” ring.
This dual-layer design allows for both localized and global energy balancing. When imbalances occur within a single module, the bottom layer handles the correction with minimal energy transfer steps. When cells in different modules are out of balance, the top layer facilitates inter-module energy flow. By reducing the average number of transfer stages, the system minimizes cumulative losses and accelerates the equalization process.
To validate the performance of their design, the researchers employed graph theory as an analytical tool. This method models the battery pack as a network, with cells represented as nodes and energy paths as edges. Each type of converter is assigned an efficiency weight, allowing the team to calculate the expected performance across various imbalance scenarios.
The analysis revealed that the average energy path length in the double-layer system is significantly shorter than in conventional single-layer ring topologies. This translates directly into faster equalization speeds and higher overall efficiency. The model predicted a 46% improvement in balancing speed compared to existing single-layer designs—a claim later confirmed through simulation.
Using MATLAB/Simulink, the team built a detailed simulation model of the 12-cell system. Each cell was represented by a 1 F capacitor with initial voltages ranging from 3.1 V to 4.2 V to simulate real-world imbalance conditions. The simulation results showed that the double-layer equalizer achieved full voltage convergence in just 0.39 seconds. In contrast, a single-layer ring equalizer under identical conditions took 0.72 seconds to reach the same level of balance. This 46% reduction in equalization time underscores the effectiveness of the hierarchical design in minimizing energy transfer latency.
But speed is only one aspect of performance. Efficiency—how much of the transferred energy actually reaches its destination—is equally critical. The researchers conducted a thorough efficiency analysis of both the Buck-Boost and switched-capacitor components. The Buck-Boost circuit, which relies on inductive energy storage, was found to have an efficiency of approximately 78.3%, primarily limited by conduction losses in the switches and inductor resistance. The switched-capacitor circuit, which transfers charge directly between cells via capacitors, demonstrated a higher efficiency of 89.5%, benefiting from fewer energy conversion stages.
By combining these two technologies in a layered architecture, the system leverages the strengths of each. The bottom layer, operating within tightly coupled modules, achieves high efficiency through short transfer paths. The top layer, managing inter-module balancing, maintains performance by minimizing the number of conversion steps required for long-distance energy movement.
To verify their findings experimentally, the team constructed a hardware prototype using four 18650 lithium-ion cells (3.7 V, 2600 mAh). The Buck-Boost circuit was implemented with IRF3205 MOSFETs, a 100 μH inductor, and appropriate control circuitry. The switched-capacitor circuit used the same switches and a 100 μF capacitor. The experimental setup allowed direct measurement of current, voltage, and switching waveforms during active balancing.
In the Buck-Boost test, two cells with initial voltages of 3.58 V and 4.00 V were connected. The measured charging current was 0.64 A, and the discharging current was 0.56 A. Using these values and accounting for equivalent series resistance, the team calculated an energy transfer efficiency of 78.31%, closely matching the theoretical prediction.
For the switched-capacitor test, the same voltage differential was applied. The charging current reached 0.7 A, and after factoring in resistive losses, the efficiency was determined to be 89.5%. These empirical results confirmed the accuracy of the analytical models and demonstrated the feasibility of the hybrid approach in real-world conditions.
Building on these component-level results, the researchers calculated the overall system efficiency. The bottom-layer ring, combining both converter types, achieved an average efficiency of 77.43%. The top-layer ring, benefiting from the higher efficiency of the switched-capacitor links between modules, reached 82.04%. When these layers are combined in the full double-layer system, the total equalization efficiency was found to be 67.32%.
In comparison, a single-layer ring equalizer using the same hybrid topology was calculated to have an efficiency of 40.54% under similar conditions. This represents a 26.78% improvement—nearly a third more efficient—thanks to the optimized energy routing enabled by the dual-tier structure.
The implications of this research extend beyond academic interest. As EV manufacturers strive to extend driving range and battery life, every percentage point of efficiency gain matters. A more efficient equalization system means less energy wasted during operation, longer battery lifespan, and improved thermal management. The faster balancing speed also allows the BMS to respond more quickly to imbalances, enhancing safety and performance during high-load conditions such as rapid charging or aggressive driving.
Moreover, the modular nature of the design makes it highly scalable. The same principles can be applied to battery packs with more cells or different configurations. The use of standardized Buck-Boost and switched-capacitor units simplifies manufacturing and maintenance, while the ring topology ensures bidirectional energy flow without the need for complex central controllers.
From a control perspective, the system offers flexibility. The Buck-Boost converters can be modulated using pulse-width modulation (PWM) to precisely regulate energy transfer, while the switched-capacitor circuits operate in discrete charge-discharge cycles. This hybrid control strategy allows the BMS to adapt to different imbalance severities—using fast, high-efficiency capacitor transfers for minor corrections and more powerful inductive transfers for larger discrepancies.
The research also highlights the value of interdisciplinary approaches in engineering innovation. By applying graph theory—a mathematical framework typically used in network analysis—to battery systems, the team was able to quantify and optimize performance in a way that traditional circuit analysis alone could not achieve. This methodological contribution opens new avenues for evaluating and designing future battery topologies.
While the current study focuses on a 12-cell system, the scalability of the architecture suggests potential applications in larger battery packs used in electric cars, buses, and even grid-scale energy storage. Future work may explore adaptive control algorithms that dynamically adjust the operation of each layer based on real-time cell data, further enhancing performance.
Another promising direction is the integration of this equalization system with advanced state estimation techniques. By combining precise voltage balancing with accurate state-of-charge and state-of-health monitoring, next-generation BMS platforms could deliver unprecedented levels of battery optimization and predictive maintenance.
The environmental impact of this technology should not be overlooked. More efficient battery systems reduce energy waste, lower operating costs, and contribute to the overall sustainability of electric transportation. With the global push toward decarbonization, innovations like this play a crucial role in accelerating the transition to clean energy.
In conclusion, the double-layer ring equalizer developed by Han Xinsheng, Kan Jiarong, and their colleagues at Yancheng Institute of Technology represents a significant leap forward in battery management technology. By rethinking the fundamental architecture of equalization circuits, they have created a system that is faster, more efficient, and more scalable than existing solutions. Their work, published in Electronic Science and Technology, demonstrates how combining established power electronics with innovative structural design can yield transformative results.
As the automotive industry continues its electrification journey, advancements in battery technology will remain a key differentiator. This research not only addresses a critical technical challenge but also sets a new benchmark for what is possible in active battery balancing. With further development and commercialization, this double-layer ring equalizer could become a standard feature in future EVs, helping to unlock the full potential of lithium-ion battery technology.
Han Xinsheng, Kan Jiarong, Ling Huiying, Wang Peng, Cheng Qian, Yancheng Institute of Technology; Electronic Science and Technology; doi:10.16180/j.issn1007-7820.2024.04.003