Novel Passive Equalization Methods for Lithium-ion Batteries Utilizing Real-Time Internal Resistance Measurements

In recent years, lithium-ion batteries (LIBs) have gained immense popularity and widespread adoption in various applications. Due to the voltage and capacity requirements of these applications, a single LIB is often inadequate. To meet these demands, LIBs are interconnected in series and parallel configurations to achieve higher voltage and capacity levels. To improve performance and prolong the lifespan of battery packs, battery equalization methods are employed to ensure balanced state-of-charge (SOC) and voltage levels among individual battery cells. Passive battery equalization methods are commonly utilized in industrial applications due to their simplicity and cost-effectiveness. The implementation of complex techniques to enhance performance may not be suitable for passive equalization methods, as it would introduce complications and increase implementation costs. This paper introduces two innovative equalization methods: the rank-sum ratio (RSR) based method and the piecewise function based (PFSO) method. These methods leverage real-time internal resistance measurement devices to enhance the equalization process. To evaluate their effectiveness, the simulation model of battery module is developed based on the Thévenin-based equivalent circuit, utilizing data obtained from hybrid pulse power characteristic (HPPC) tests. Simulation results demonstrate that the proposed methods outperform other passive equalization methods, including the SOC-based and voltage-based methods. In a setting with a 16-cell battery module possessing a total capacity of 51.2V and 104Ah, the RSR and PFSO methods can equalize the battery module, adjusting the capacity from 98.97Ah to 102.56Ah and 101.12Ah, respectively, as opposed to the 100.90Ah and 100.09Ah achieved by the SOC-based and voltage-based methods.