Protocol-Aware Threshold-Tuned Passive Balancing for Lithium-Ion Battery Packs: Experimental Validation of Voltage Uniformity and Thermal Safety

Abstract

Maintaining voltage uniformity and thermal stability in series-connected lithium-ion battery packs is essential for ensuring performance, safety, and cycle life, particularly in electric vehicle applications. This study presents an optimized, threshold-based switched passive balancing strategy implemented through MOSFET activation. The objective is to evaluate balancing effectiveness under both constant current (CC) and constant current–constant voltage (CC-CV) protocols and to determine the optimal activation scheme. The proposed system was experimentally validated on a six-cell (21.6 V, 2.6 Ah) Li-ion module. The balancing logic dynamically initiates charge redistribution when cell voltage disparities exceed a defined threshold (V_min). Results show that under CC charging, balancing significantly reduced voltage deviation while maintaining comparable charge duration. Under CC–CV charging, delayed balancing activation during the low-current CV phase led to improved charge equalization, sustained voltage alignment across cycles, and enhanced thermal stability. In all cases, power dissipation was maintained below 1 W, confirming the energy-efficient operation of the design. Unlike conventional passive balancing, we experimentally demonstrate a protocol-aware, threshold-tuned strategy that delays activation into the CV phase to minimize heat while maximizing equalization. The design maintains total dissipation below 1 W and sustains cross-cycle alignment under CC–CV, establishing a practical operating envelope for EV packs. Compared to conventional fixed-threshold passive balancing, the proposed protocol-aware scheme achieves lower voltage deviation, sub-1 W power loss, and superior thermal stability during CC–CV charging. Unlike prior passive balancing schemes, this work provides the hardware-validated, protocol-aware, CV-delayed balancing framework, experimentally proven to sustain ΔV ≤ 45 mV across 50 cycles with < 1 W dissipation. This establishes a new benchmark for scalable battery management system (BMS) deployment in next-generation EV and grid storage systems.