Stable Configuration of a Li-Ion Series Battery Pack Based on a Screening Process for Improved Voltage/SOC Balancing

Abstract

Differences in electrochemical characteristics among Li-ion cells inevitably result in state-of-charge (SOC) imbalance when the existing voltage balancing technique is used. This paper presents a new approach based on a screening process for improved voltage/SOC balancing of a Li-ion series battery pack. Two kinds of screening processes, capacity screening and resistance screening, are implemented in an orderly manner. The capacity screening process matches open-circuit voltage (OCV)-SOC relation for achievement of voltage/SOC balancing. In the resistance screening process, pulse-type discharging/charging currents are applied to various SOC points to select the cells with similar voltage variance. Through two screening processes, the cells that have similar electrochemical characteristics are finally selected, and can be used for stable configuration of a Li-ion series battery pack. The proposed screening process has been validated by extensive experimental results and SOC estimation results based on extended Kalman filter.