High accuracy state-of-charge estimation of Li-Ion battery pack based on screening process

This paper presents a high accuracy state-of-charge (SOC) estimation of a Li-Ion battery pack. Specifically, to overcome the difference in electrochemical characteristics among Li-Ion cells and to improve accuracy of SOC estimation, screening processes for the selection of the cells that have similar electrochemical characteristics has been previously implemented to improve accuracy of SOC estimation. Using the screened cells, battery packs with cells in series, parallel and series-parallel connection can be easily configured and safely used. The capacity, open circuit voltage (OCV) and two resistances (R_i, R_Diff) of each battery pack are measured and compared to the average unit cell. Through the parameter comparison, each model of all battery packs can be simplified into a unit cell model. Using simplified models for battery packs, all SOC estimates based on extended Kalman filter (EKF) are within ±5% of the values estimated by ampere-hour counting.