Influence of Battery Downsizing and SOC Operating Window on Battery Pack Performance in a Hybrid Electric Vehicle

This paper provides a parametric study for (1) downsizing a battery pack (reducing the number of battery cells), potentially reducing cost and weight; and (2) lowering the nominal operating SOC to reduce degradation. The downsized pack design with shifted SOC window is evaluated in a lightduty hybrid electric vehicle (HEV) where the power demanded by the battery pack is specified prior to downsizing. A calibrated electro-thermal model and a semi-empirical capacity fade model are used to capture voltage, state-of-charge, temperature and capacity loss of the downsized battery. The capacity fade model is developed based on a novel set of experiments designed to clarify the influence of nominal operating SOC on battery degradation. The parametric study shows that the pack size could be reduced from 76 to 64 cells while shifting nominal operating SOC from 50% to 35% without experiencing battery power denials. This would result in a 20% increase in energy utilization per cell, with only a 0.4% increase in capacity fade.