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

State-of-charge (SOC) operating windows strongly affect lithium-ion battery degradation, while conventional aging tests require long durations to establish trends. Coulombic efficiency (CE), defined as the discharge-to-charge capacity ratio, provides an early-life diagnostic for parasitic reactions and long-term performance prediction. Eight 21700 NMC cells were cycled at 25 °C across four SOC windows (0–100%, 20–80%, 40–60%, and 80–100%) using conventional and ultra-high precision cyclers. Capacity retention, resistance growth, and CE were evaluated to quantify depth-of-discharge (DOD) effects. A non-linear aging behavior was observed, with accelerated initial capacity loss followed by stabilization. The 0–100% SOC window exhibited the highest degradation, with ~9% capacity loss per 100 EFC initially, stabilizing to ~3.3% per 100 EFC, corresponding to a projected 80% SOH life of ~440 cycles. In contrast, the 40–60% window showed stabilized fade of only 2.0% per 100 EFC, yielding a projected life of ~2670 cycles (~6× improvement). CE stabilized near unity (≈0.998–1.000) within the first several cycles. Small deviations (e.g., 99.95% vs. 99.99%) revealed irreversible side reactions. Cumulative inefficiency after 30 cycles was lowest for partial SOC windows and highest for full-range cycling, correlating strongly with long-term degradation trends.