Thermal Performance Evaluation and Design Optimization of a Battery Disconnect Unit for High-Power Electric Vehicle Applications

The battery disconnect unit (BDU) is one of the most essential safety systems and battery control sub-assemblies within conventional electric vehicles (EVs) to date. The BDU plays a critical role in electro-mechanical control and protection by using contactors and fuses to manage the flow and distribution of power. The selection of busbar size and means of convection is critical given the restrictive mechanical constraints imposed by OEM battery tray designs-including requirements for vibration, electrical isolation, manufacturing tolerances, and creepages and clearances. This study aims to provide a thermally focused evaluation of various busbar thicknesses in addition to also determine whether natural convection is sufficient to manage the heat rise, or if forced convection is required using a combined joule heating modelling approach in MATLAB Simulink and computational fluid dynamics (CFD) with SolidWorks. A mixed drive cycle and constant current charging conditions were applied to determine key hotspots and assess component temperature rise. The methodology incorporates mechanical constraints, high voltage schematics, and justification of electrical component selection to provide a practical and thermally reliable custom BDU. Results showed that 6 mm thick C110 copper busbar limited the temperature rise under a DC fast charging (DCFC) case of 29.09° C whereas thinner busbars exceeded 30° C. Forced convection with a convection coefficient of $35\ \mathrm{W} /\left(\mathrm{m}^{2} \cdot \mathrm{K}\right)$ was superior to natural convection with a ROA for the DCFC case being 4.35° C as opposed to the natural convection case of 13.27 °C, both approaches maintained temperatures within acceptable limits. Ultimately, natural convection was selected due to its thermal adequacy and drastically lower mechanical complexity.