Battery thermal management systems (BTMS) are crucial for ensuring the safety, performance stability, and longevity of electric vehicle (EV) batteries. Among various cooling approaches, static single-phase immersion cooling has attracted attention due to its structural simplicity and potential for passive operation without active circulation. This study experimentally evaluates the thermal performance and intercellular temperature uniformity of cylindrical battery modules immersed in three dielectric fluids: deionized water, RT22HC, and mineral oil. A 24-cell battery simulator was subjected to heat loads ranging from 10 to 50 W in three configurations: immersion alone, immersion with finned heat pipes, and immersion with heat pipes combined with forced convection. Performance was assessed using maximum temperature, cooling effectiveness, and the Temperature Uniformity Coefficient (TUC). At 50 W, deionized water achieved the highest cooling effectiveness (34.22%), while RT22HC demonstrated superior temperature uniformity (TUC ≈ 0.010). Dimensionless analysis reveals that deionized water and RT22HC operate in a similar Rayleigh regime (∼107), but different Prandtl numbers govern the jet structure and intercellular heat redistribution. These results suggest that the fluid performance hierarchy is controlled by the Rayleigh Prandtl regime, not just individual thermophysical properties. These findings provide a scaling-based physical interpretation of static single-phase immersion cooling and support its potential as a low-complexity BTMS solution for mid-scale EV applications. © 2026 Elsevier Ltd.