Battery pack topology structure on state-of-charge estimation accuracy in electric vehicles

- Analyze the effect of battery pack topology structure to SOC deviation.
- Battery inconsistency changes regularly due to battery pack topology structure.
- Obtain the equivalent relation of the dynamic inconsistency to the inherent inconsistency.
- Discuss the relationship between the leakage current and the input impedance.
- Propose an improved schematic diagram for battery module voltage sampling.

Battery inconsistency are exacerbated by the resistance of inter-cell connecting plate (RICP) and the input impedance of battery voltage monitoring system (IIBVMS), both of which cannot be eliminated in a battery pack. Experiments show that the inconsistency distribution and SOC deviation in a battery pack is restricted by RICP and IIBVMS. In this paper, a “parallel-series” battery pack model is proposed and validated. The effects of RICP and IIBVMS on SOC deviation are analyzed. Results indicate that RICP causes unevenly current flowing through each in-parallel battery cell, and so the battery cell directly connected to the battery module posts presents the lowest SOC. In order to restrict SOC deviation to an acceptable level, the equivalent relation of the battery inconsistency caused by RICP to the inherent inconsistency is discussed. The reasonable matching RICP is then presented. IIBVMS causes different current leakage in each voltage sampling line. The closer the battery module connects to the positive terminal of a battery pack, the higher leakage current is. The different leakage current in each voltage sampling line exacerbates battery inconsistency and causes different initial SOC for each battery module. The relationship among the leakage current, the input impedance and the number of in-series battery modules are discussed. And an improved schematic diagram of battery module voltage sampling is further presented to lower the leakage current and to solve the sampling time delay caused by the multi-switch.