Evaluating isothermal degradation error in lithium-ion batteries under WLTP drive cycles

Physics-based battery management systems frequently assume isothermal cell oper-ation, eliminating the thermal sub-model to reduce computational cost. The error introduced by this assumption in state-of-health prediction depends on the activation energy of the solid electrolyte interphase kinetic rate constant. In widely used pa-rameterisations this energy is set to zero, decoupling temperature from degradation kinetics and concealing the true thermal sensitivity. A calibrated multi-mechanism Single Particle Model with Electrolyte is developed for nickel manganese cobalt oxide and lithium iron phosphate cells. A two-stage sequential procedure calibrates the ac-tivation energy from temperature-dependent fade rate ratios, then jointly optimises the solid electrolyte interphase rate constant and loss of active material coefficient against a longitudinal state-of-health trajectory, achieving a root mean square er-ror of 1 . 45 pp. — 6 . 3-fold improvement over the uncalibrated default. A parametric sweep of 24 conditions spanning both chemistries, ambient temperatures of 10, 25, and 40 °C, and charge rates of 1C and 2C shows that the isothermal assumption in-troduces state-of-health errors of 1 . 4–8 . 2 pp. (for nickel manganese cobalt oxide) over 500 Worldwide Harmonised Light Vehicle Test Procedure cycles with the calibrated activation energy of 78 . 5 kJ mol −1 , compared with a maximum of 0 . 20 pp. at zero ac-tivation energy. For nickel manganese cobalt oxide cells, solid electrolyte interphase growth accounts for >99% of capacity loss. For lithium iron phosphate cells, solid electrolyte interphase and loss of active material contribute approximately 62% and 38% respectively, due to the larger depth of discharge per cycle. A three-layer parameter uncertainty analysis one-at-a-time sensitivity, an activation energy sweep across 62 – 95 kJ mol −1 , and an analytical Monte Carlo, confirms that the isothermal Δ SOH error exceeds 0.5 pp. across the full physically plausible parameter space and that activation energy is the dominant source of uncertainty.