Thermal Management of Electric Vehicles

As electric vehicles (EVs) become more advanced, research to find longer lasting, cheaper, and more powerful EVs has taken off. Automotive companies are starting to develop electric vehicles in hopes of replacing gas operated vehicles in the future. Companies such as Mini (BMW), Volvo, and Cadillac have committed to only selling EV cars in the next 10 years [1]. 

Figure 1: Electric Vehicles

Electric motors are considered one of the major concerns in thermal management of electric vehicles. The heat generation in electric motors is critical, as overheating could damage the motor components such as the insulation, magnets, or bearings. Therefore, having proper thermal management will significantly improve the safety and efficiency of electric vehicles.   

Research conducted by McMaster University looked at a variety of electric motor cooling solutions to address thermal management. One of the proposed solutions to the heat generation of motors was to produce a polymer-potting compound product to lower motor operating temperature (also known as axial cooling) [2]. The graph below shows the baseline cooling and the axial cooling method in comparison.

Figure 2: Axial cooling vs Baseline cooling at peak temperature [2]

As shown in Figure 2, the axial cooled peak temperature is about 20°C lower than the traditional baseline cooling. Additionally, it was found that as the thermal conductivity of the polymer potting compound increases, the operating temperature of the motor decreases. There is high potential for a high thermal conductivity potting compound, but this area needs further development.

Nowadays, most EVs use lithium-ion batteries. Lithium-ion batteries have high energy density and a long life-cycle. Although they are becoming more powerful, these batteries can have significant disadvantages. If overheating of the battery is not properly addressed, in a worst scenario, thermal runaway can cause serious damage to the product and potential injuries to the users. Multiple EV companies have reported cases in which uncontrolled thermal runaway resulted in a vehicle fire. Hence, thermal management is essential to address the thermal issues of electric batteries, motors, and power electronics.

Thermal management also helps in performance and improves EV battery life. Thus, researchers developed multiple battery thermal management systems (BTMS) to mitigate the temperature of EV batteries. Methods such as using phase change materials (PCMs), conductive packaging, thermal interface materials (gap pads, thermal grease), and forced air cooling have been explored.

PCMs are a passive cooling method, as they can absorb heat around the battery module. “The heat will be stored in the form of latent heat and the increasing temperature will be minimized [3].” Highly conductive materials such as graphite have been used in PCMs and achieved low battery temperature. A PCM’s thermal conductivity is an important attribute in  the overall PCM’s effectiveness.

Figure 3: PCM in a nutshell

In a study by the Chemical Engineering Department at Tarbiat Modares University, Tehran, scientists looked at enhancing the temperature control of batteries by using phase change materials and aluminum wire mesh plates. Polyethylene Glycol 1000 (PEG1000) was used as a phase change material. Aluminum wire mesh plates were used to enhance thermal conductivity. PEG 1000 has a thermal conductivity of 0.23 W/mK and the aluminum mesh has a thermal conductivity of 203 W/mK [4]. The highly conductive aluminum helps with heat dissipation from the cell surfaces to the PCM. The chart below shows the temperature profiles of the battery during the discharge rate of 3°C [4].

Figure 4: Cell temperature profiles during the discharge rate of 3°C [4]

As seen in Figure 4, battery cells with PCM have better thermal management. The cell surface temperature with no PCM is 62.5°C. The battery cell with PCM and aluminum wire mesh plates showed a 25% reduction in temperature to 46.5 °C, which results in a better battery efficiency [4]. In conclusion, thermal management can noticeably reduce the surface temperature of the battery cell during operation.

Another integral application of thermal management is for thermal interface materials (TIMs). TIMs are inserted between two or more surfaces to assist thermal conduction between materials. Adhesives, potting compounds, and ceramics are examples of thermal interface materials.

A study from Advanced Materials Interfaces [5] used copper nanowires for TIMs. Since copper is cheaper than silver and gold, with a high bulk thermal conductivity, it makes the ideal material for TIMs. Figure 5 shows a SEM image of copper nanowires.

Figure 5: SEM image of copper nanowires

The thermal conductivity of copper nanowires was measured at 30°C and 80°C for over 100 hours, as they intended to analyze the long-term performance of copper nanowires as TIMs (Figure 6).

Figure 6: Thermal conductivity of nanowires at 30°C and 80°C

Developers also use heat sinks for thermal management in electric vehicles. Materials such as aluminum and copper alloys are common heat sink materials. C-Therm measured the thermal conductivity of pure aluminum foil with a thickness of 0.13mm. The TPS (Transient Plane Source) Slab utility on the Trident Thermal Conductivity Instrument was used. Figure 7 shows the experimental setup.

Figure 7: FLEX TPS Slab Utility Set Up

Figure 8: Thermal Conductivity results

The data is summarized in Figure 8 above. The sample matched with the reference thermal conductivity values of aluminum within 1%. This shows the ability of Trident’s FLEX TPS Slab utility to test thin, conductive samples.

Figure 9: MTPS Sensor with Liquid Cell

The C-Therm MTPS (Modified Transient Plane Source) method is ideal for characterizing the thermal conductivity of PCMs, as it has the ability to test the solid and liquid states of a material continuously. The MTPS method is used to characterize:

• Phase Change Materials (PCMs)
• Polymer Composites
• Battery Components
• Metals
• Fluids/Pastes
• Ceramics
• And many more

Thermal Analysis Labs, a division of C-Therm, offers a wide range of thermal analysis testing. For more information on our testing services, please contact us directly at info@thermalanalysislabs.com, or call (506)457-0498.

Written by Jason Ho, Laboratory Technician