Effects of Working Fluids’ Properties and Battery Spacing on Battery Temperature in Single-phase Static Immersion Cooling System

In this study, the single-phase static immersion cooling for lithium-ion batteries was theoretically and experimentally investigated. To predict the battery temperature, a theoretical model was developed based on the lumped capacitance method and the Bernardi equation, considering state of charge (SoC) and C-rate conditions during charging. Experiments were conducted using three types of dielectric fluids and nine battery spacings ranging from 0.6 mm to 6 mm using dummy batteries capable of controlling the SoC and C-rate conditions. The experimental results showed that the theoretical model agreed with experimental data within a deviation of 6%. Based on the results, fluid viscosity exhibited the most significant influence on the cooling performance of the single-phase static immersion cooling. Especially, lower viscosity enhanced natural convection, thereby reducing the battery temperature more effectively than other fluid properties such as thermal conductivity or volumetric heat capacity. Additionally, the battery spacing was identified as a critical design parameter. In particular, a rapid decrease in battery temperature was observed in the transition region where the gap ranged from one to two times the thermal boundary layer thickness, attributed to the shift from natural convection with buoyant channel flow to external natural convection. These results provide practical insights for optimizing fluid selection and battery layout, as well as chip-board configurations in data centers, including CPUs, GPUs, and related components, to enhance thermal management in single-phase static immersion cooling systems.