Control of Film Growth in Lithium Ion Battery Packs via Switches

Recent advances in lithium ion battery modeling suggest degradation may be reduced by permitting unequal but controlled charging of individual cells. Hence, this paper compares anode-side film formation for a standard equalization scheme versus unequal charging through switches controlled by deterministic dynamic programming (DDP) and DDP-inspired algorithms. A static map for film growth rate is derived from a first-principles battery model adopted from the electrochemical engineering literature. Using this map, we consider two cells connected in parallel via relay switches. The key results we demonstrate are: (1) Enabling unequal charging indeed reduces film buildup; (2) A near optimal control method can be implemented using heuristic rules designed from the DDP solution and convexity properties of film growth rate. Simulation results indicate that the heuristic rules achieve near optimal performance relative to the DDP solution, with significant reduction in film growth compared to charging both cells equally.