Estimating the Health and State of Charge of Each Cell in a Second-Life Battery System from Field Data

Effective use of battery storage depends on reliable estimation of its state of health (SOH) and state of charge (SOC). Model-based state estimation requires the open-circuit voltage (OCV) curve, which is typically unknown for second-life batteries. We present a framework that jointly estimates the states and parameters of an equivalent circuit model solely from field operation data, using Gaussian process regression to reconstruct the OCV curve. Applied to a real second-life battery system of 27 modules and 324 cells, it reveals SOH heterogeneity, a systematic SOC imbalance, and two faulty cells, all validated against a reference measurement. We aggregate the cell SOH and SOC to module level and benchmark them against a lumped-module model fitted without the individual cell voltages. The lumped-module model follows the average behavior and cannot capture the limiting cells, overestimating SOH by up to 31% and SOC by up to 23%.