Effective thermal management is crucial for maintaining the safety, performance, and lifetime of lithium-ion batteries (LIBs) in electric vehicles (EVs), particularly during high-current charge-discharge cycles that generate significant heat. This paper presents an experimental investigation and dimensionless theoretical analysis of a single-phase static immersion cooling system for cylindrical battery modules, a configuration rarely explored in the literature. Three dielectric fluids with different thermophysical properties, namely deionized water, RT22HC, and mineral oil, were tested in three cooling configurations: pure immersion, immersion with heat pipes (natural convection), and immersion with heat pipes combined with forced convection. A total of 24 cylindrical heat sources were tested at heat loads of 10–50 W, and key dimensionless parameters such as Rayleigh (Ra), Grashof (Gr), and Nusselt (Nu) numbers were calculated. Results show that increasing the heat load strengthens the natural convection due to buoyancy and increases the Nu value, with deionized water providing the most effective cooling. The integration of heat pipes and forced convection reduces the battery temperature by up to 34.22% (deionized water), which mathematically lowers the Nu value but significantly reduces the total thermal resistance of the system. This study is the first to validate the classical laminar Nu-Ra correlation (exponent = 0.246) for vertical cylindrical cells in a static immersion system, linking experimental findings and dimensionless modeling. The results provide practical guidance in fluid selection and configuration optimization for compact, passive, and energy-efficient battery thermal management systems in EV and stationary energy storage applications. © Published under licence by IOP Publishing Ltd.