Low-Complexity Fast Charging Strategies Based on Explicit Reference Governors for Li-Ion Battery Cells

This article proposes and compares a new family of low-complexity control schemes for the fast charge of lithium-ion (Li-ion) battery cells accounting for degradation constraints. These schemes are based on a two-level architecture, where a low-level linear quadratic regulator (LQR) ensures stability and fast tracking of the applied reference, while an outer control layer, based on an explicit reference governor (ERG), enforces constraints satisfaction by manipulating the applied reference of the lower level. The ERG is based on the construction of a suitable Lyapunov level set contained inside of linear constraints. The main challenge to build a performing ERG for the fast charge of Li-ion batteries is how to choose a convenient Lyapunov function for nonconvex constraints arising from the electrochemical model of the battery. This article proposes and compares four different approaches to obtain performing Lyapunov functions for a concave constraint evaluating the impact on the charging time of a Li-ion battery and on the required computational time. Experimental validations on commercial lithium-cobalt oxide (LCO) battery cells (Turnigy nano-tech 160 mAh) show a tradeoff between the four methods computational time and the charging time.