Design of Dielectric Fluid Immersion Cooling System for Efficient Thermal Management of Lithium-Ion Battery Packs

Heat generation during fast charging and discharging of lithium-ion batteries (LIBs) remains a significant challenge, potentially leading to overheating, reduced performance, or thermal runaway. Traditional battery thermal management systems (BTMS), such as air-based cooling and indirect liquid cooling using cold plates, often result in high thermal gradients—both vertically within cells and horizontally across battery packs—especially under high-current discharge rates. To address these issues, this study introduces and evaluates a steady-state convection-based ester-oil immersion cooling (EOIC) technique for LIBs. Numerical simulations based on the Newman, Tiedemann, Gu and Kim model, aligned with multi-scale multi-dimensional principles, were performed on both a single 18650 cylindrical cell and a 4S2P battery pack. Experimental validations were conducted under 2C and 3C discharge rates at 25°C ambient temperature. The EOIC system demonstrated a temperature reduction of up to 13°C in the 18650 cell and 15°C in the 4S2P pack at 3C discharge compared to natural air convection and achieved ≤ 10°C thermal gradient across cells in the battery pack. The simulation results closely matched experimental data, with a maximum deviation of only 2°C, confirming the model's reliability. Moreover, EOIC outperformed conventional mineral oil-based immersion cooling in both cooling effectiveness and temperature uniformity. These findings confirm EOIC as a promising passive BTMS approach, ensuring improved safety, performance, and thermal stability for LIBs in electric vehicle applications.