Advanced Battery Management Technologies for Electric Vehicles

7Battery Balancing

7.1 Background

Electric vehicles (EVs) require a battery system to provide sufficient operating voltage, power, and energy. A Li‐ion battery system in EVs generally consists of many battery packs connected in series and/or parallel, and a battery pack is made of series‐connected cells or parallel‐connected cells. In the following, the focus is on balancing issues in the battery pack with the series‐connected cells. Due to manufacturing tolerances and different operating temperatures and aging speeds, each cell in the battery pack with the series‐connected cells may differ in its terminal voltage, capacity, and internal resistance and experience an unbalanced charging and discharging, where the stronger cells in the pack can be overcharged or the weaker cells in the pack can be over‐discharged. After many cycles of charging and discharging, the imbalance in the battery pack deteriorates, which leads to safety issues, capacity loss, and reduced cycle life of cells in the battery pack and eventually reduced cycle life of the whole battery system. Battery balancing methods are developed to solve the unbalance problems in Li‐ion battery systems. These balancing methods generally include battery sorting, passive balancing, and active balancing [1].

Battery sorting can alleviate cell imbalance in a battery pack during the initial stage of making it [2–4]. The purpose of battery sorting is to carry out screening tests for the selection of the cells which possesses ...