Energy optimal operation of hybrid battery systems and comparison to a single-cell reference system for electric vehicles including the aged state of high energy cells

Abstract

In battery electric vehicles the battery system has to satisfy demanding requirements. On the one hand, high power capabilility has to be provided for varying states of charge and temperature. On the other hand, the battery system should achieve a maximum service life, optimal energy efficiency and a high overall driving range. This paper compares a conventional single-cell battery system with two hybrid battery system topologies that aim at improving the mentioned aspects of the conventional system. The hybrid battery systems each comprise two battery parts connected via a dc-to-dc converter, one part including lithium-ion battery cells optimized for high energy density and the other part equipped with high power density type lithium-ion cells. An optimal control problem is formulated and solved with Dynamic Programming. The mentioned topologies are then compared in terms of energy efficiency, power capability and current distribution. A real life driving cycle power demand is used for this comparison. It can be shown that the hybrid battery system reduces the occurrence of high current rates on the high energy battery part and shows better energy efficiency when operated with aged high energy battery cells. Furthermore the hybrid battery system's potential to obtain a high power capability in low state of charge regions is shown.