RIT Racing Battery Cell Characterization and Test Fixture

The RIT Formula SAE Team, also known as RIT Racing, designs, manufactures, and builds a small open-wheel style race car (like a small Formula 1 car) to compete against other colleges at official SAE competitions around the world. The team started in 1991 as an internal combustion team and made the switch to an electric vehicle in 2019. With a long history of competing at a high level, most recently this past season as winners of the North American competition, the team hopes to continue to advance their technologies to stay competitive. Through the development of a battery cell test fixture, the team can gain valuable insight into how the individual battery cells will perform within the vehicle’s battery. While some suppliers provide data sheets with ample data such as internal resistance, charge-discharge rate, and operating temperature, many suppliers provide data sheets with minimal usable data for the team’s use case. The fixture has the ability to collect array of thermal values, cell expansion pressure, charge-discharge, test voltage, current, and internal resistance. Validating data from the sheet as well as collecting data beyond that will be done to be able to optimize future battery designs. The fixture consists of a pressure fixture to compress the cell, an enclosure for the fixture to ensure safety, custom circuit boards to collect data, a power supply, and an output display to view the data. The mechanical fixture is an aluminum base plate with two garolite (a type of flame-retardant fiberglass) plates that the cell sits in between, with load cells in between the aluminum plate and the bottom garolite plate to measure the force exerted by the expansion of the battery cell. Four nuts and bolts are used to compress the three plates and the cell together, transferring the load measurement into the load cells. The tabs on the battery cell, where the electricity flows in and out of the cell, are attached to bus bars that go to the power supply as well as sensors for measurement. This fixture is then enclosed in a safety enclosure, which will contain the cell if it were to combust during testing. The enclosure is made out of steel and has a flash-rated viewing panel. The enclosure also has a vent on it for any off-gases during testing, as well as to prevent pressure from building. There are multiple circuit boards used to make the fixture work. There is a discharge board, the charge board, and the Sensors and Thermistor Array Board (STAB). The discharge board is designed to drain the excess energy from the cell using the charge to power LEDs. The charge board regulates the amount of power going into the cell from the power supply. The STAB has a thermistor array on it making it able to measure the temperature gradient across the cell surface as well as an array of other sensors to measure inputs from the load cell, the voltage of the cell, and the current of the charge/discharge circuit. This board is adhered to the back of a garolite plate on the fixture with the thermistors poking through the garolite to be able to measure the heat gradient across the cell. The firmware for the battery test fixture acts as the controller for the tests. It reads all the different sensors on the fixture, acts as a watchdog for potentially dangerous conditions, and is a liaison for data going between the hardware and the companion app. The data from the sensors is transferred to the STM microcontroller which then outputs it to the companion application. The companion application allows the user to set up and monitor tests and process the resulting data. This application is designed to run on any computer that can be connected to the test fixture via USB. Through the companion application, the user can quickly configure a test using preset templates and monitor the data while the test is running to ensure the test is proceeding as intended. This fixture will be used for many years to help optimize the battery pack for the FSAE car made by RIT Racing. This will lead the team to build a better car, helping them continue their successful legacy and help them create a more data-driven design for the battery