Beyond SOH: A Composite Degradation Index with Uncertainty-Aware SOC Estimation for EV Batteries

Conventional battery health indicators such as State of-Health (SOH) capture only capacity fade, while neglecting operational stress, round-trip efficiency losses, and model-derived uncertainty. All of these features accelerate aging in real world electric vehicle (EV) environments. Extending our prior uncertainty-aware State-of-Charge (SOC) estimation work, this paper introduced a novel degradation index named the Unified Degradation Score (UDS), a Ridge-regularized composite metric integrating Depth-of-Discharge (DOD), SOH, C-rate, Round-Trip Efficiency (RTE), and Monte Carlo (MC) Dropout epistemic uncertainty. Validated on the CALCE CS2-36 dataset
over 729 controlled aging cycles, the underlying FT-Transformer achieved MAE 0.599% and RMSE 0.846% during early-life operation. Beyond cycle 250, epistemic uncertainty rose systematically, functioning as an automatic model-derived degradation signal.The UDS achieved a Pearson correlation of r = −0.975 with SOC prediction error, outperforming SOH alone (r = −0.938).A closed-loop BMS charging simulation demonstrated that UDS triggered current throttling preserved 79.0% remaining useful life (RUL) at cycle 729, surpassing conventional SOH-step control (77.2%) and unmitigated fixed-rate operation (64.1%).