64 September/October 2025 | E-Mobility Engineering Peter Donaldson examines the complexities of fluid-based thermal management in highenergy battery systems Cool chargings As batteries pack more energy into a given mass and volume while charging speeds increase, engineers face growing challenges in managing heat dissipation efficiently and safely. In wrestling with this problem, they are reshaping thermal management strategies while navigating the complex considerations involved in selecting cooling media and distribution systems, and balancing the thermal, electrical and mechanical properties of materials. Modern batteries generate significantly higher heat loads than did their predecessors of just a few years ago. The shift toward larger cells has reduced their surface-area-to-volume ratios, which makes heat rejection during fast charging more difficult. To address this problem, some manufacturers are adopting immersion cooling techniques using dielectric fluids. These fluids enable direct contact with cell surfaces, allowing heat to be removed more efficiently while minimising temperature gradients within cells and across the pack. Another innovative solution now entering production is the use of flexible cooling ribbons that conform to cell geometries, eliminating gap fillers, adhesives and thermal interface materials. Light and slim, they increase energy density and reduce the amount of coolant needed, while enlarging the area in contact with cylindrical cells thanks to a larger wrapping angle. This improves thermal management in fast charging and in extreme ambient temperatures. The constant push for higher efficiency and reduced weight has driven innovation in thermal interface materials, including advanced adhesives and gap fillers. These materials play a vital role in bonding cells and conducting heat away from critical areas. New cell configurations and substrate materials require continuous adaptation of these solutions, leading to continuous improvement and innovation. Another major trend is the integration of cooling systems directly into battery packs, moving away from external cooling plates. This shift has spurred the development of specialised dielectric and low-conductivity coolants that can safely interact with electrical components. Faster charging demands systems that can handle high peak thermal loads during rapid charging while avoiding excessive cooling in normal operating conditions. This has given rise to advanced cooling architectures, including multi-functional heat exchangers and predictive control algorithms. Safety is a perennial priority, with stricter regulations pushing for better thermal runaway mitigation. New simulation tools are being employed to model heat propagation and optimise cooling strategies, particularly for lithium iron phosphate, future chemistries and solid-state designs, which exhibit varied heat generation profiles. Fluid choices The choice of cooling fluid is a critical decision that balances thermal performance, system complexity, cost and safety. Several options are available, each with distinct advantages and limitations. Water–ethylene glycol (WEG) solutions have become the industry standard for many applications, and are familiar to engineers from internal combustion applications. They offer excellent heat transfer properties, are cost-effective and work well in closedloop systems. Weakly conductive or dielectric WEG variants reduce electrical risk in case of leaks, making them a safer option for battery packs. Battery thermal runaway CFD simulation with Simcenter STAR-CCM+ illustrating gas diffusion details from cylindrical cells (Image courtesy of Siemens Simcenter)
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