Hybrid balancing in a modular battery management system for electric-drive vehicles

A hybrid balancing approach that combines active and passive balancing is presented for a modular battery management system (BMS) in electric-drive vehicles. The hybrid system combines advantages of module-level active balancing with cost-effectiveness of cell-level passive balancing. A module consists of a number of cells connected in series, with cell-level passive balancing performed within a module, together with a module-level dc-dc converter that performs active balancing among the modules. Furthermore, the module-level dc-dc converters also supply auxiliary vehicle loads on the vehicle low-voltage bus, thus removing the need for a separate high-voltage to low-voltage dc-dc converter. System control and communication are shared among active and passive balancing functions, reducing the system complexity. Design details and experimental results are provided for a hybrid balancing system implemented on a lithium-ion battery pack, which consists of eighteen 25 Ah cells grouped in three 6-cell modules, each including a 480 W dc-dc converter supplying power to the vehicle low-voltage bus.