Effect of terminal resistance and SoH asymmetry on parallel cell interaction and battery system degradation

In large-scale battery systems, differences in internal and terminal resistance among individual cells induce current imbalance and circulating currents, collectively referred to as parallel cell interaction (PCI). These phenomena accelerate localized degradation, shorten system lifespan, and increase operational risk. However, most prior studies have focused on simplified module-level analysis without addressing the root causes of PCI or its long-term impact under realistic operating conditions. To bridge this gap, this study develops and experimentally validates a high-precision equivalent circuit model (ECM)-based parallel battery simulator combined with a novel resistance measurement method. The model accurately reproduces voltage and current behavior, achieving root mean square errors of 8.9 mV and 0.057C, respectively, thereby enabling quantitative PCI analysis. Using this simulator, the study systematically quantifies the impact of terminal resistance asymmetry and state-of-health (SoH) imbalance under realistic EV driving schedules. A 10 % resistance mismatch in a 2-parallel configuration reduced system lifespan by up to 3.2 %, with an additional 0.8 % loss per added parallel branch. These results demonstrate that PCI-induced degradation cannot be mitigated by resistance or SoH monitoring alone, and establish current distribution balance as a key indicator of system stability. To address this, a practical strategy involving passive resistance adjustment is proposed to redistribute current, improve cell utilization, and synchronize degradation. Unlike conventional methods, this approach specifically resolves intra-branch imbalances. Its simplicity and compatibility with routine maintenance highlight its potential to enhance thermal safety, extend battery life, and reduce total cost of ownership in EV and stationary storage systems.