Functional Observer Design for Parallel Connected Li-Ion Battery: A Descriptor Systems Theory Approach

This letter investigates cell-level state of charge (SOC) estimation in a parallel connected lithium-ion (Li-ion) battery pack under a reduced sensing scenario. The internal states of individual cells within a battery pack are likely to differ due to cell inconsistencies caused by manufacturing tolerances and usage conditions. This creates a need for cell-level estimation in a parallel connected battery pack. The dynamics of parallel connected battery packs require solving differential algebraic equations (DAEs). This letter considers the reduced sensing scenario, measuring only the total current and voltage for estimating individual cell SOCs and local currents in a battery pack. A novel functional observer, designed under milder conditions, is proposed and the observer design is formulated in terms of a feasible linear matrix inequality (LMI) problem. In the simulation, the SOC of two cylindrical Li-ion battery cells are estimated, and the results illustrate a fast convergence and hence the effectiveness of our approach.