Effects of supercapacitor self-discharge on the performance of battery– supercapacitor hybrid energy storage system

The complementary feature of batteries and supercapacitor (SC) in terms of energy density and power density makes their hybrid energy storage system (HESS) an effective solution in applications requiring a high-power density and high energy density. However, intrinsic limitations like self-discharge of SC can affect HESS performance. Most studies for HESS use oversimplified models for SC with constant self-discharge values. To investigates the effect of SC self-discharge on the performance of a HESS, this study uses an intelligent data driven model for SC that uses variable leakage resistance to account for self-discharge with good accuracy. Experimental validation confirmed the accuracy of the model used in predicting voltage profiles during charging/discharging and SC self-discharging. The results show that SC self-discharge considerably diminishes HESS energy efficiency, particularly during prolonged idle times or low peak group duty ratios. For instance, higher peak power demands (e.g., 4 W) improve efficiency due to reduced SC idle time, while lower peak group duty ratios (e.g., 5.56 %) exacerbate losses from self-discharge. In addition, the voltage decay of SC during idle intervals is accurately modeled only when variable leakage resistance is included, highlighting the nonlinear nature of self-discharge. Furthermore, delayed charging of the SC (initiated just before peak demand) reduces energy losses by 29% compared to instant charging, as it minimized the duration when SC remains at high voltages where self-discharge rates are highest. For practical applications, these findings emphasize the importance of adaptive energy management strategies that account for SC self-discharge, especially in systems with intermittent high-power demands. Future work could explore real-time leakage resistance estimation or hybrid charging protocols to further optimize HESS performance.