Cell Balancing Control For Lithium-Ion Battery Packs: A Hierarchical Optimal Approach

Effective cell equalization is of extreme importance to extract the maximum capacity of a battery pack. In this article, two cell balancing objectives, including balancing time reduction and cells' temperature rise suppression, are taken into consideration simultaneously. Furthermore, hard constraints are imposed on the cells' state-of-charge levels, currents, and equalizing currents. Based on a developed module-based cell-to-cell balancing system model, a multiobjective constrained optimization problem is formulated, which aims at the coordinated control of all equalizers rather than individually controlling the equalizer for its two adjacent cells' equalization. Next, a hierarchical cell equalizing control approach is proposed, where the module-level controlled equalizing currents are first designed at the top layer, and then, the cell-level equalizers are controlled for each battery module in parallel at the bottom layer. The designed hierarchical structure significantly reduces the computational burden, making the cell equalizing algorithm more implementable in real time. Following the Lyapunov stability analysis, the convergence of the designed cell equalizing control algorithm is proved. Illustrative results demonstrate that the balancing time can be reduced by up to 29.8% compared with the decentralized equalizing control.