The Thermal Management of Li-ion Battery Packs

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Given the long development cycle for vehicles, automobile manufacturers must plan their upcoming lines far in advance. With growing emission regulations and the rising cost of gas, full electric and hybrid vehicles are becoming more attractive and growing in market share. At the Fiat Research Center in Orbassano, the focus is on developing electric and hybrid vehicles using lithium and lead-acid batteries as well as supercapacitors. Fiat currently has several light trucks that run on electric drives, and the next application will be an electric version of the Fiat 500, which has been announced for the US market. The challenge lies in combining as many as 100 lithium-ion battery pouch cells into battery packs that generate the 350V needed while providing sufficient cooling and keeping the packs as small and light as possible. The maximum temperature differential among all the cells in a pack must not exceed 5 °C. If the temperature of the pack is too low, it limits the charge you can extract; if it is too high, it risks thermal runaway, leading to electrolyte emission, smoke, or fire.

Fiat, an established automobile manufacturer, is focusing on the development of electric and hybrid vehicles at its Research Center in Orbassano, near Turin, Italy. The company is working on integrating lithium and lead-acid batteries as well as supercapacitors into their vehicle designs. Fiat has already introduced several light trucks with electric drives and is planning to launch an electric version of the Fiat 500 for the US market. The research team, led by Senior Research Engineer Michele Gosso and coordinated by Antonio Fiumara, is responsible for developing power solutions for these electric and hybrid vehicles. Their work involves not only the design and integration of battery packs but also ensuring these packs are efficient, safe, and adaptable to various vehicle models.

To address the challenge of cooling multiple battery pouch cells, Fiat's research team does not manufacture the individual lithium-ion battery cells but combines up to 100 of them into battery packs. They use COMSOL Multiphysics to study the surface distribution of heat and prefer convection air cooling. Their model divides each surface of the pouch cell into nine areas, corresponding to thermocouples on the cell itself. This allows them to examine the temperature distribution at various charge/discharge rates and verify the model's consistency with reality using thermocouples and infrared heat cameras. This approach provides invaluable information that cannot be achieved through other means due to the difficulty of embedding thermocouples in battery pouches. With the knowledge gained from the model, Fiat has been able to reduce the physical channels between the cells, cutting down on space and weight. This makes it easier to insert the battery pack into a larger variety of vehicles. Additionally, they determined that a less powerful fan was required, reducing costs. The model also helped cut design time by 70%, from 1,000 hours to roughly 300 hours. Future projects will look at extreme conditions, such as temperatures below freezing, leveraging the Joule heating effect and innovative design to solve charging difficulties in these conditions.

• The use of COMSOL Multiphysics allowed Fiat to study the surface distribution of heat in battery packs, providing invaluable information that cannot be achieved through other means.
• The model helped reduce the physical channels between the cells, cutting down on space and weight, making it easier to insert the battery pack into a larger variety of vehicles.
• Fiat determined that a less powerful fan was required, which helped reduce costs.

• Design time reduced by 70%, from 1,000 hours to roughly 300 hours.