Dielectric Strength of Electric Vehicle Fluids (Lubricants and Coolants) at Usual Operating Temperatures

The development of suitable fluids, including lubricants or coolants, for electric vehicles (EVs) requires large efforts on their formulation, characterization, and performance testing. An important property for these fluids is dielectric strength or breakdown voltage (BDV), which is expected to be as high as possible at wide temperature range. Short circuits and bearing currents are highly undesirable phenomena which are known to cause inefficiencies and potential failure or wear of electromechanical hardware if BDV magnitude is not adequate. In modern EVs, the typical fluids comprise lubricants (for bearings and gears in the driveline) such as automatic transmission fluids (ATFs) and low-viscosity gear oils, and coolants (for thermal management in the battery pack and power electronics), namely, ethylene-glycols and dielectric oils. Although these fluids are reported to have acceptable BDV at room temperature (according to ASTM D877 and/or D1816 standard methods) for electric equipment, BDV is likely to be altered when temperature changes occur. The information of BDV at room (20-30°C) temperature is often reported for dielectric fluids but it is scarce for other temperatures. Knowing the BDV at wide range of temperatures is valuable and currently wanted to better understand and predict the performance of EV fluids in harsh environments. To contribute with new data of BDV for typical and modern EV fluids at wide temperature range, this work aims to evaluate the BDV of 10 different EV fluids (comprising lubricants and coolants) under the ASTM D877 standard method in the range from 10 to 70 °C.