Comparison of immersion and side cooling systems of cylindrical Li-Ion cells

Heat Transfer and Fluid Flow Laboratory, Faculty of Mechanical Engineering, Brno University of Technology, Technicka 2896/2, 616 69 Brno, Czechia

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Published online: 12 March 2026

Abstract

The safety and durability of Li-Ion batteries strongly depend on effective thermal management, especially under high C-rate operation. This work presents a comparative numerical investigation of immersion and side cooling applied to cylindrical 18650 cells. A transient three-dimensional model was developed for a series-connected pack of nine cells subjected to a 3.8 C discharge rate. Heat generation was implemented through a source term defined as a function of temperature and discharge time, enabling a realistic yet computationally straightforward representation of electro-thermal behaviour. The two cooling strategies were systematically assessed with respect to coolant flow rate, focusing on maximal temperature and temperature spread across the pack. The results show that both immersion and side cooling achieve nearly identical thermal performance in terms of maximal temperature and temperature spread. However, side cooling suffers from a higher pressure drop, whereas immersion cooling places stricter demands on the sealing and dielectric properties of the coolant. Overall, the study demonstrates that each method has specific trade-offs, and the optimal choice depends on the balance between thermal performance, hydraulic losses, and design complexity.