Impact of battery cell imbalance on the voltage behavior of commercial Ni-MH EV/HEV battery modules

Electric and hybrid-electric vehicles’ (EV/HEV) battery packs typically are composed of hundreds of single battery cells, which are usually grouped in modules and installed in a pack to collectively power EV/HEV. Due to nonidentical characteristics of battery cells, the imbalance of cell voltage, cell capacity and state of charge (SOC) could potentially lead to over-charge or over-discharge batteries, which may shorten the lifetime of the whole on-board battery pack. In this research, we used a commercial Ni-MH battery module from a hybrid Honda Insight/Civic to investigate the influence of cell imbalance on the battery cell SOC and voltage behavior. A series of experiments were designed and carried out to explore the impact of different cell arrangements and operation conditions. Experimental results indicated that the degraded cells could directly reduce the module output performance. It is found that battery cells would take longer time to stabilize their working voltage in the charge-discharge operation when they were connected to the degraded cells. It was also found that cell imbalance could not only influence the SOC of healthy cells, but also significantly reduce the capacity utilization efficiency of the whole battery module.