Fast Charging-Minimum Degradation Optimal Control of Series-Connected Battery Modules with DC/DC Bypass Converters

This paper presents a multi-objective fast charging-minimum degradation optimal control problem (OCP) for a Lithium-ion battery series-connected module with DC/DC bypass converters. Each cell in the module is modeled via coupled nonlinear electrochemical, thermal, and aging dynamics. Due to the multiscale temporal nature of the model, a surrogate model for the solvent diffusion aging dynamics is developed to mitigate prohibitive simulation time. The multiobjective OCP is formulated to find a trade-off policy between fast charging and degradation decay. For this purpose, the direct collocation approach is utilized to transcribe the OCP to a nonlinear programming (NLP) problem by parametrization of the system states, inputs, and charging times. The proposed OCP is formulated according to two different schemes: (i) same-charging-time (SCT) and (ii) different-charging-time (DCT). The former assumes simultaneous charging for all cells, whereas the latter determines different charging cell times. The performance of the proposed SCT and DCT schemes is validated on an illustrative case study of a battery module with two series-connected cells in the presence of initial state of charge imbalance.