Analysis and Comparative Study of Different Converter Modes in Modular Second-Life Hybrid Battery Energy Storage Systems

The use of extransportation battery system (i.e., second-life electric vehicle/hybrid electric vehicle batteries) in grid applications is an emerging field of study. A hybrid battery scheme offers a more practical approach in second-life battery energy storage systems, because battery modules could be from different sources/vehicle manufacturers depending on the second-life supply chain and have different characteristics, e.g., voltage levels, maximum capacity, and also different levels of degradations. Recent research studies have suggested a dc-side modular multilevel converter topology to integrate these hybrid batteries to a grid-tie inverter. Depending on the battery module characteristics, the dc-side modular converter can adopt different modes, such as boost, buck, or boost-buck to suitably transfer the power from the battery to the grid. These modes have different switching techniques, control range, different efficiencies, which give a system designer choice on an operational mode. This paper presents an analysis and comparative study of all the modes of the converter along with their switching performances in detail to understand the relative advantages and disadvantages of each mode to help to select the suitable converter mode. Detailed study of all the converter modes and the thorough experimental results based on a multimodular converter prototype with hybrid batteries have been presented to validate the analysis.