This week, the FSAE Electric Racecar’s batteries subteam finished designing the battery box. Designing the battery box was a major challenge because of the rules imposed by the Formula SAE Electric competition. For example, the rules require that the battery box be able to withstand at least 20g acceleration in any direction, be removed from the racecar to be charged, and made of fire resistant material (minimum UL94-V0). With all these restrictions, it is impressive that the batteries team was able to create a rules compliant design while reducing the overall weight of the battery box by about 100 pounds compared to the 2016-2017 battery box. With the design for the battery box finally completed, the batteries subteam has ordered new batteries and begun manufacturing.

This week, Christopher Tracey will be explaining how he designed a cooling system for the battery box.

This year, the FSAE Electric Racecar team opted for lithium-ion batteries, the same kind Tesla uses in all their cars. Lithium-ion batteries have much better power density (they hold more energy while weighing less) than traditional chemistries like lead-acid or nickel metal hydride batteries. Lithium-ion batteries also don’t have some of the drawbacks such as charge memory, a characteristic of nickel metal hydride batteries. However, Lithium-ion batteries are sensitive to temperature extremes and must remain in a relatively narrow temperature window. Too cold, and they don’t deliver power well. Too hot, and they create a dangerous chain reaction explosion.

Naturally, batteries in a high-draw environment, like powering a racecar, will get hot. The cooling system is designed to make sure these batteries never reach a temperature where they will explode, for both safety and reliability reasons. Different cooling solutions were considered, like heat sinks and liquid cooling. The optimal solution, due to its weight, practicality, and cost, was a forced convection system that ventilates the entire battery box. The cooling system will draw cool, ambient air in through the batteries, over the heat-sensitive electronics, and exhaust out the top of the box.

Different combinations of fans, battery arrangements, and fan placements within the box were discussed and tested with SolidWorks flow simulation to evaluate the designs’ effectiveness. The design had to meet the cost, size, and cooling requirements, meet IP65 standards , and conforming to other team members’ designs (it has to fit!). The best solution was to use 3 fans, one for each section, depicted in red above. Each one is rated at 225 cubic feet per minute, powered by a separate low-voltage power supply.