Comparison of Cyclic Aging and Coulombic Efficiency Measurements for Varied SOC Windows in Lithium-Ion Batteries.

State-of-charge (SOC) operating windows critically influence the long-term performance and reliability of lithium-ion batteries. While conventional cyclic aging studies across varying SOC ranges provide valuable insights into capacity fade and resistance growth, they often require extended testing periods to produce meaningful trends. Coulombic Efficiency (CE), defined as the ratio of discharge to charge capacity per cycle, has emerged as a promising early-life diagnostic tool for identifying parasitic side reactions and projecting long-term degradation. In this work, we present a comparative study of cyclic aging and CE behavior across multiple SOC windows (0–100%, 20–80%, 40–60%, and 80-100%) using eight 21700 NMC cells cycled at 25 °C on both a conventional cycler and an ultra-high precision cycler. Capacity retention, internal resistance evolution, and CE profiles were analyzed to establish correlations between SOC window and degradation rates. Results are expected to indicate that Depth of Discharge (DOD) affects both observed aging rates and the predictive sensitivity of CE as an early indicator of long-term performance. By tracking CE with high precision during the first few dozen cycles, even tiny deviations from 100% CE (e.g., 99.95% vs. 99.99%) can reveal parasitic side reactions such as SEI growth, lithium plating, or electrolyte decomposition. These early patterns correlate strongly with long-term degradation trends, enabling faster and more accurate predictions of cycle life and highlighting CE’s potential to accelerate lifetime testing and enhance predictive modeling for automotive and stationary energy storage applications.