Multi-Loop Control of Hybrid Li-ion Battery Packs Using the Auxiliary DC Bus for State-of-Charge Regulation

Energy storage systems comprising hybrid Li-ion battery packs that fully utilize the energy density and power density of different lithium-ion (Li-ion) chemistries can achieve reduced weight and volume with improved lifetime. A capacitively-coupled architecture has been proposed and demonstrated to realize a hybrid battery-energy storage system design that combines energy-dense and power-dense battery packs. However, charging the power-dense pack is a challenge due to the DC blocking nature of the capacitive coupling. In this paper, a multi-loop control of the DC-DC converters in the active battery management system (BMS) is proposed to maintain the average state-of-charge (SOC) of the power-dense pack while balancing cells within each pack. The detailed control loop analysis and design are presented. Experimental results from a 500 W prototype BMS hardware comprising two 250 W isolated dual active bridge (DAB) converters are provided to demonstrate the stable operation and dynamic performance of the proposed multi-loop control design under different operating modes and transients.