38 degraded the composite’s tensile qualities. Further investigations used a combination of Paraffin-20 and Caprylone, an organic ketone, as PCMs provide a notably different thermal performance due to their distinctive latent heat profiles. The higher melting temperature of the paraffin mixture, despite its slightly lower latent heat capacity compared to Caprylone, could lead to a more uniform temperature. The battery module used in this experiment is made up of nine parallelarranged (1S9P) 21700 lithium-ion battery cells. All the cells are spotwelded to nickel strips and copper wires to form a single-positive and a singlenegative module tab for charging and discharging processes. The module case was made using MEKP (Methyl Ethyl Ketone Peroxide) and Ripoxy R-804 Vinyl Ester Resin. Two comparable PCM cell cases were also manufactured, each with 20% wt Paraffin-20 mix or the Caprylone PCM. A higher amount of the PCM was tested but the PCM was no longer fully enclosed by the resin of the case, and thus failed the repeated cycle test due to the PCM leaking out. Then, 5% wt graphite (G) powder was added to the composite mixture to enhance its overall conductive heat transfer. All of these materials were heated in a water bath at 60 °C until all the PCM melted, slowly stirred at approximately 60 rpm for two minutes, and then poured into a 74 mm × 74 mm rectangular mould specifically designed to contain nine 21700 battery cells in a 3 × 3 configuration, with the smallest distance between cylindrical cells being 2 mm. The composite was then cured for at least 24 hours before its removal from the mould, resulting in a solid battery holder. The contact part with the battery was smoothed with sandpaper before an Arctic MX4 thermal paste (less than 1 g for each module) was thinly applied on its surface to ensure maximum contact and heat dissipation possible from the battery cells to the module. The total weight of each battery module composite is 75± 2 g. Two separate specimens with the same composition, with and without carbon fibre as a reinforcement material, were built to evaluate their mechanical properties. The mechanical properties of the composite are typically compromised by the inclusion of paraffin, where the ability to resist mechanical stresses is nearly 70% lower than the composite without PCM. Graphite has the benefit of creating a more evenly distributed mixture and improving overall heat transfer, but it also further reduces the maximum force and stress. Unfortunately, it is still challenging to fabricate a reinforced PCM composite battery module, particularly properly fitting the carbon fibre in between the battery cells. Further developments to investigate the module strength under other direct mechanical abuses, such as object penetration and long-term vibrations, as well as vehicle crashes, are ongoing. Overall, both paraffin and Caprylone can be used as a PCM for passive protection against any potential thermal abuses in EV battery modules, while in terms of mechanical strength, the use of a composite reinforcement material is strongly encouraged. Inorganic salts Another type of PCM is based on inorganic hydrated salts, which are salts in water solution. Generally, it is a saturated solution and initiates crystallisation at phase change temperature. This potentially provides an ideal phase change behaviour as it melts isothermally similar to a pure material. These have low thermal expansion, good latent heat and thermal conductivity, are non-toxic and are nonflammable. But they are corrosive and the phase change reversibility is challenging to retain for a longer duration. The temperature for the operation of the system must agree with the PCM phase change temperature. In case of Li-ion battery packs it is in the range of 20–25 C. To gather all the heat created by the system, the PCM must have high latent heat and high thermal conductivity for allowing rapid transfer of heat and its storage. This is determined using differential thermal analysis (DTA) and differential scanning January/February 2024 | E-Mobility Engineering Different styles of cooling fins on PCM cells (Image courtesy of Research Center for Transportation Technology)
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