Formula Student Germany: Keeping Electric Race Car Batteries Safe with iButton

Fomula Student Germany is a design competition that has its roots in the Formula SAE competition that the Society of Automotive Engineers (SAE) began in the U.S. in 1981. Since 1988, SAE and the Institution of Mechanical Engineers in England have been hosting annual Formula Student competitions.

In this contest, students are challenged to build a single-seat formula race car that’s used in competition against teams from around the world. It’s not just the fastest car that wins—also considered are construction, performance, and financial and sales planning efforts. Teams approach the event as if they are an automotive manufacturer developing a prototype to be evaluated for production. The cars should demonstrate good acceleration, braking, handling, and dependability and should also be offered at a reasonable cost.

Challenges

  • Ensure safe operation of race car batteries

Solutions

Benefits

  • Easy, accurate temperature monitoring of battery cells
  • Small iButton devices fit in space-constrained race car accumulator

Formula Student Germany participants build an electric, single-seat formula race car for competition against teams from around the world. Photo courtesy of Formula Student Germany. Formula Student Germany participants build an electric, single-seat formula race car for competition against teams from around the world. Photo courtesy of Formula Student Germany.

Challenges

Safety is the top priority for Formula Student Germany. As such, the student competitors must ensure that the batteries used to power their electric cars are stored properly and at the right temperature. These batteries are stored in an accumulator container on the vehicle. Each accumulator must be monitored by an accumulator management system (AMS) when the car is active or the accumulator is connected to a charger. The AMS must provide continuous measurement of:

  • All cell voltages
  • The tractive system current
  • The temperature of thermally critical cells
  • And, for lithium-based cells, the temperature of at least 30% of the cells equally distributed within the accumulator container

According to the competition rules, cell temperature must be measured at the negative terminal of the respective cell. The sensor used must be in direct contact with either the negative terminal or less than 10mm away from the terminal on the respective busbar. To meet the requirements established for the competition, Formula Student Germany sought a solution that would allow the students to easily and accurately monitor the safety of their batteries as well as their compliance to the temperature rules.

Solution and Benefits

For temperature monitoring, Formula Student Germany chose the DS1922T iButton® temperature logger with 8KB data-log memory. The DS1922T measures temperature at a user-defined rate, recording the results in a protected memory section. It provides temperature accuracy of ±0.5°C from +20°C to +75°C.

“We chose the iButton because it is easily implemented, as it does not need any connector or external power supply,” said Sarah Battige, who provides electrical inspection from the Formula Student Germany Operative Team. “It is also very robust, so it can handle the harsh environment of a race car with the high amount of vibrations and the raised temperature in the accumulator. It is also not influenced by electromagnetic interference caused by the motor controllers. Additionally, the iButton is quite a small device, so it can easily be installed in the tight space of the race car’s accumulator.”

After the competition, data logged by the iButton was quickly and easily retrieved for inspection of each team’s compliance with the battery cell temperature rules.

This temperature-over-time plot from one of the Formula Student Germany teams shows the data collected on their race-car battery from the DS1922T iButton temperature logger. This temperature-over-time plot from one of the Formula Student Germany teams shows the data collected on their race-car battery from the DS1922T iButton temperature logger.