Advanced Thermal Management Systems for Electric Vehicles

Electric Vehicles (EVs) are celebrated as a green energy solution paving the way for a pollution-free future. At the heart of modern EVs lies the lithium-ion battery pack, which presents significant challenges due to its sensitivity to temperature variations. This article delves into the current battery thermal management systems (BTMS), assessing their practical applications and design considerations to ensure thermal safety and explore international market potential.

The article conducts an in-depth thermal stability analysis, addressing critical issues such as:

• Thermal runaway scenarios in extreme weather conditions
• The impact of sub-zero temperatures
• Varying charging and discharging rates
• Short circuit failures in EVs
• Mechanical deformation of equipments

Given these challenges, optimising BTMS is crucial for the widespread adoption of electric vehicles. Effective thermal management not only extends battery lifespan but also enhances overall safety. In industry, EV batteries are considered to have reached the end of their useful life once they experience a 20% capacity loss or a 30% increase in internal resistance. Engineers rely on active and passive BTMS to combat battery overheating and ensure optimal performance.

How can we ensure safety in a Fully Electric and Hybrid Vehicle Battery Pack?

Ensuring safety in fully electric and hybrid vehicle battery packs is crucial, especially since consumer confidence hinges on the reliability of these systems. Lithium-ion (Li-ion) batteries, the most commonly used type, have faced criticism for their instability and potential fire risk. Li-ion cells use volatile organic solvents, like ethylene carbonate and ethyl methyl carbonate, as electrolytes. To address these concerns, standards have been established to account for heat generation from Rechargeable Energy Storage Systems (RESS), specific crash-test requirements, and protection against short circuits. However, given the diverse range of product designs, no single standard can cover all safety aspects, leading to potential unforeseen issues.

Thermal management for lithium-ion batteries is not just a theoretical concern—it’s been extensively researched and validated. Here are some key findings:

• Drake et al. (2015) conducted theoretical and practical investigations into the impact of temperature on lithium-ion batteries’ charging and discharging rates, revealing that substantial heat generation adversely affects battery performance.
• Zhang et al. (2017) addressed the challenges associated with the fast charging lithium-ion batteries, identifying elevated temperatures as a significant issue. Many other researchers have echoed these findings, underscoring the critical importance of thermal regulation in maintaining battery efficiency and longevity.

• Rodriguez-Martinez et al. (2014) found that prismatic pouch cells offer better thermal management due to their large surface area, which aids heat dissipation and is cost-effective.

The need for effective thermal management systems in EVs is clear. By optimising these systems, we can significantly enhance battery lifespan and safety, making electric vehicles a more reliable and attractive option for consumers.

Read More: Electric Car Thermal Management to Prevent Catching Fire

Thermal Stability of Batteries for BTMS

Thermal runaway (TR) occurs when a battery generates more heat than it can dissipate, potentially leading to catastrophic outcomes. Mechanical, electrochemical, or thermal failures can trigger this. Researchers agree that mechanical abuse of lithium-ion batteries, without proper safety measures, can lead to thermal runaway. Electrical failures, such as short-circuiting and overcharging, also contribute to TR by increasing current flow and power dissipation, causing the battery to heat up. As temperatures rise, electrolyte oxidation can cause the battery to swell, exacerbating the problem. Thermal failures caused by loose cell packaging or increased internal resistance can also lead to TR. Efficient heat dissipation can stop internal exothermic reactions, but if the cathode reacts and the separator melts, a voltage drop occurs, making TR unavoidable. As temperatures rise above 60°C-100°C, electrolyte reduction accelerates, further increasing temperatures to 160°C-170°C.

A notable incident involving a thermal runaway occurred with the Boeing 787 Dreamliner. The NASA Engineering and Safety Center (NESC) addressed the safety concerns, recommending manufacturers design batteries to prevent such catastrophic events.

Impact of Temperature Fluctuations on the Performance of Li-ion Batteries

Temperature changes can greatly affect how lithium-ion batteries perform, especially in cold conditions. When the temperature drops, the chemical stability of these batteries can decrease, cutting their power and energy capacity by up to 40%. This is due to factors like the battery’s design, shape, and the effectiveness of the electrolyte solution in cold weather. Studies have shown that pouch-type batteries lose capacity faster as temperatures rise. For example, their discharge capacity at 25°C is higher than at 0°C or 45°C.

Battery Thermal  Management Strategies

To manage battery temperatures, a thermo-electric model can be used to study how heat is distributed in cylindrical 18,650 battery cells. This model shows that the temperature rises more during the discharge cycle than the charging cycle, although this difference lessens with higher current rates (C-rate). The way a battery heats up is related to changes in entropy and the current applied. A detailed analysis has found that high current flow can suddenly stop the discharge or charge process, making the battery more likely to overheat and potentially experience thermal runaway during discharge.

Advanced Cooling Techniques and Configurations for Optimizing BTMS

Research using neural networks has shown that higher discharge rates increase the surface temperature of batteries. Key factors in effective battery cooling include maximum temperature, space utilisation, and energy efficiency. Optimal cooling performance is achieved when the airflow intake and outlet are positioned at the top of the battery pack. However, cooling efficiency decreases as the number of batteries increases horizontally.

Studies on tube-shell battery packs revealed that baffles improve fluid contact and heat transfer by altering the air-fluid flow. For convective cooling, the arrangement of lithium-ion batteries is crucial. In staggered arrays, the highest temperature increase is proportional to the longitudinal interval with a given airflow rate, whereas in aligned arrays, it’s the opposite. For both setups, increasing the transverse interval raises the battery temperature.

Various BTMS strategies have been reviewed, including phase change materials (PCM), phase change fluids, flexible PCMs, hybrid cooling, air and liquid cooling, and heat pipe-based cooling. The flammability of organic PCMs and the stability of inorganic PCMs are critical for improving BTMS. Yue et al. (2021) explored a hybrid cooling approach for Li-ion battery packs, combining micro heat pipes, forced air convection, and intermittent water spray. Their global sensitivity analysis found that the thermal conductivity and thickness of the PCM layer, the heat pipe’s length, and the water’s inlet velocity significantly affect the battery pack’s maximum temperature and temperature distribution. 

Authors conclusion

Effective thermal management systems (BTMS) are essential for safely and efficiently operating lithium-ion batteries in electric vehicles (EVs). This article has highlighted the critical challenges posed by temperature variations, including thermal runaway and the impact of sub-zero temperatures on battery performance. We can significantly improve battery safety and longevity by exploring advanced cooling techniques and configurations, such as phase change materials and hybrid cooling systems. Optimising BTMS enhances battery life and makes EVs more reliable and appealing to consumers, ensuring a sustainable and pollution-free future.

Author: Mohd Aman, Arindam