Development and validation of a modular active balancing system for electric vehicle batteries

• Main Author: Lee, Wai Chung
• Published: University of Bristol 2013
• Subjects: 629.2293
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.627935

The characteristic mismatch among the series connected cells/modules of an electric vehicle (EV) battery has a negative impact on the overall battery performance. This thesis covers the research work of developing an active battery balancing system that can perform balancing at both cell and module level for an EV battery in order to counter the mismatch. The work includes the design of the system, building the corresponding prototypes and the experimentation of the prototypes circuits. The prototype cell and module balancing circuits have been tested experimentally with lithium-ion polymer batteries, and a Hardware-in-the-loop (HIL) simulation system that was developed for this research to emulate the electrical behaviours of batteries for more effective testing. The prototype active cell balancing circuit is capable of balancing a four-cell battery module, where one of the cells is initially 10%-SOC higher than the others, to only 1 % deviation after balancing for 52 minutes at a rate of 0.11C. Comparing to the conventional passive balancing method, the prototype active cell balancing circuit can recover the discharge capacity and energy of an imbalanced module by 6%. For a six-module battery pack where the capacity of one module is 10% less than those of the others, the module balancing circuit can improve the discharge capacity and energy by 5%. The module balancing circuit also manages to balance the modules in a more diverse condition where a 2% SOC deviation exists initially between every two consecutive modules of a six-module battery pack. This overall 10% SOC deviation can converge to 1 % after balancing for 117 minutes at a rate of 0.11 C - 0.12C. The results obtained from the prototype cell and module balancing circuits validate the feasibility of the conceptual design and demonstrate their balancing capabilities, and the abilities of improving the degree of energy utilization of an imbalanced battery.