Mechanism and mitigation of temperature nonuniformity in CO2 direct-cooled battery packs during fast charging by evaporating pressure matching

The rapid advancement of fast-charging technology imposes significant challenges on battery thermal management. Dynamic mismatch between refrigerant heat transfer and battery thermal inertia in direct cooling systems leads to poor temperature uniformity under high heat generation during fast charging. To address this problem, this study developed and experimentally validated a multi-physics coupled simulation model of a CO2 direct cooling battery thermal management system. The model particularly focuses on battery temperature uniformity under fast charging conditions, with emphasis on the coupled flow–thermal response under transient heat generation. Then, the issue of poor temperature uniformity due to the dynamic mismatch between refrigerant heat transfer and battery thermal inertia was identified. To improve temperature uniformity, the steady-state temperature distribution characteristics were studied under various conditions. A multi-parameter feedforward evaporation pressure control strategy was proposed. The simulation results indicate that as battery heat generation increases, the battery temperature uniformity significantly deteriorates, and the temperature gradient is concentrated near the cold plate outlet. Additionally, under 1.5C charging conditions, the EPC strategy maintains an average battery temperature of 30 °C while reducing the maximum temperature difference by 46.9% during charging. By improving temperature uniformity without hardware modification, the proposed method increases the fast-charging thermal safety margin and extend the cycle life of vehicle batteries.