Impact of C-rates on Electrical Behavior of Li-ion Batteries under Coupled Temperature-load Conditions

The utilization of lithium-ion batteries in electric vehicles becomes increasingly common. Battery performance degradation is influenced by both temperature and boundary constraints. Monitoring the usable cell capacity (UCC) under real-world operating conditions is essential for ensuring accurate state-of-health estimation. Capacity tests in laboratory settings are typically conducted at low C-rates to approximate equilibrium conditions, whereas in real vehicle applications, charging currents are often much higher. This discrepancy in charge/discharge rates frequently results in deviations between laboratory characterization and on-board BMS capacity estimation. To investigate the impact of C-rates under coupled temperature-load conditions, we conducted long-term cycling tests on LFP/graphite pouch cells at 25 °C and 45 °C under different constrained conditions. The UCC was evaluated at different test rates (1/20C, 1/3C, and 1C) as an indicator of aging. The results show that significant capacity recovery occurs during high-rate cycling at 25 °C with the magnitude of recovery increasing with higher C-rates. The UCC increases to around 107% of the initial capacity at around 600 cycles under 25 °C and 1C cycling. Meanwhile, battery aging can be divided into three stages with different aging speed at low-rate test. The influence of boundary constraints is negligible at low rates but is markedly amplified under high rates. Similarly, high temperature accelerates the transition to the accelerated-aging stage, whereas high C-rates further magnify the temperature effect. Mechanistic insights were obtained from the Christensen-Newman model, differential voltage analysis (DVA) and direct current internal resistance measurement, to elucidate the degradation characteristics under different C-rates. The analysis reveals that high-rate capacity recovery originates from inter-layer inhomogeneity along the electrode thickness, which causes incomplete lithium utilization. This manifests in DVA as an initial increase followed by a decrease in the available lithium inventory. These findings highlight the distinct degradation characteristics under different test rates, providing a reference for reconciling laboratory standard tests with on-board BMS capacity estimation.