Study of Multiple Energy Storage Units Under A System Architecture of Reduced Quantity of Power Converters

Proper handling of retired electric vehicles (EVs) batteries is essential for safeguarding the environment. Although these batteries are no longer suitable for EVs, they can be repurposed as second-life batteries (SLBs), providing few more years of service in stationary storage applications before complete disposal is necessary. An SLB energy storage system (ESS) includes SLB packs, battery, and energy management systems (BMS & EMS), and associated power conditioning systems (PCSs). Traditionally, the PCS architecture for N units of SLBs includes an equal number of DC/DC converters cascaded up to the rated voltage of an upstream DC/AC converter (inverter). However, the necessity of this arrangement can be reconsidered if a proper coordination strategy is established between the inverter and the remaining converters. According to Kirchhoff's Current Law (KCL), the sum currents flowing into and out of a node must be equal to zero. This implies that the SLB string without a converter should hypothetically meet the remaining current requested by the upstream inverter Therefore, this research aims to investigate the fundamental challenge of removing a DC/DC converter from the conventional PCS setup that comprises of equal number of converters and batteries, while ensuring the reliable operation of different SLB clusters through strategic power flow control between the inverter and the remaining converters. Various scenarios that involving different current flow directions and voltage differences between the inverter, DC-link, DC/DC converter, and SLBs, were also used for the validation of simulation study in MATLAB/Simulink environment. Results indicate that removing a DC/DC converter from the typical PCS architecture is not feasible. This is because the battery unit without the DC/DC converter has become a dominant load, continuously absorbing current/charging rather than supplying to the DC-link and inverter. This ultimately leads to system failure due to battery overcharging and potential thermal runaway.