[7] Ouyang, Q., Ghaeminezhad, N., Li, Y., Wik, T., & Zou, C. (2024). A unified model for active battery equalization systems. IEEE Transactions on Control Systems Technology (accepted). arXiv preprint arXiv:2403.03910.

In this work, we introduce a novel, hypergraph-based approach to establish the first unified model for various active battery equalization systems. This model reveals the intrinsic relationship between battery cells and equalizers by representing them as the vertices and hyperedges of hypergraphs, respectively. With the developed model, we identify the necessary condition for all equalization systems to achieve balance through controllability analysis, offering valuable insights for selecting the number of equalizers. Moreover, we prove that the battery equalization time is inversely correlated with the second smallest eigenvalue of the hypergraph's Laplacian matrix of each equalization system. This significantly simplifies the selection and optimized design of equalization systems, obviating the need for extensive experiments or simulations to derive the equalization time. 

[6] Skegro, A., Zou, C., & Wik, T. (2023). Analysis of potential lifetime extension through dynamic battery reconfiguration. 25th European Conference on Power Electronics and Applications [Link].

This simulation study analyzes the potential of dynamic reconfiguration for extending battery lifetime w.r.t. several parameters. Results indicate that the lifetime extension is larger for series than for parallel configurations. For the latter, the dominant factor is equivalent full cycles spread at the end of life, but resistance increase with age and the number of cells in parallel are also influential. Finally, for the former, the number of series-connected elements amplifies these effects.

[5] Han W., Wik T., Kersten A., Dong G., & Zou C., (2020).  Next-generation battery management systems: Dynamic reconfiguration, IEEE Industrial Electronics Magazine, 14(4), 20-31. [PDF]

In fixed configurations, battery system performance is, in principle, limited by the weakest cells, which can leave large parts severely underutilized. Allowing the dynamic reconfiguration of battery cells, on the other hand, enables individual and flexible manipulation of the battery system at cell, module, and pack levels, which may open up a new paradigm for battery management. Following this trend, this article provides an overview of next-generation BMSs featuring dynamic reconfiguration. 

[4] Tang X., Yao, K., Zou, C., Gao, F., Hu, W., Xia, Y., ... & Liu, B. (2022). U.S. Patent No. 11,316,212. Washington, DC: U.S. Patent and Trademark Office. [PDF]

[3] Tang X., Zou C., Wik T., Yao K., Xia Y., Wang Y., Yang D., & Gao F., (2020). Run-to-run control for active balancing of lithium iron phosphate battery packs. IEEE Transactions on Power Electronics, 35(2), 1499-1512. [PDF]

In [3-4], we addressed the weak observability problem inside the voltage plateau and the computational issue caused by real-time state estimation for the management of lithium iron phosphate batteries. The technical novelties first arise from the introduction of a balancing current ratio (BCR) based algorithm and its combination with a voltage-based algorithm, separately responsible for the balancing task within and beyond the voltage plateau. Then, the battery balancing process was innovatively formulated as a batch-based run-to-run control problem, in which the control policy was implemented in two timescales, i.e., timewise and batch-wise. The proposed algorithm requires a small space, only 118 kilobytes in C language, and is very cheap to implement online. Furthermore, it can extract 97.1% of the theoretical pack capacity and is 5.7% more efficient than the conventional voltage-based algorithms.

[2] Han W., Zou C., Zhou C., & Zhang L., (2019). Estimation of cell SOC evolution and system performance in module-based battery charge equalization systems. IEEE Transactions on Smart Grid, 10 (5), 4717-4728. [PDF]

[1] Han W., Zou C., Zhang L., Ouyang Q., & Wik T., (2019). Near-fastest battery balancing by cell/module reconfiguration. IEEE Transactions on Smart Grid, 10(6), 6954-6964. [PDF]