Optimizing battery thermal management in race cars

Building an industry standard race car in one year

Ecurie Aix was founded in 1999 when students at RWTH Aachen University in Aachen, Germany had the idea for creating a Formula Student team. Formula Student is a worldwide competition where over 500 academic teams build their own car per the regulations of the SAE or the Formula Student Germany (FSG). The goal of the competition is to have the students design, manufacture and test a race car in one year. The Ecurie Aix team started building the cylinder engine (ICE) driven car in 1999 and during the summer of 2022 it will race with its 10th fully electric car.

Each Formula Student event consists of static and dynamic disciplines. Static disciplines include an engineering design event, cost event and a business plan presentation. The dynamic disciplines are tests involving endurance, efficiency, autocross, acceleration, skid pad and track drive. The most important discipline in each competition is the endurance race, which is 22 kilometers (km) that is split in two parts (each 11 km).

Ecurie Aix was looking for an efficient and reliable approach for the thermal and electric characterization of battery packs and understood they needed to combine and integrate several disciplines due to the complexity of the problem. By partnering with Siemens Digital Industries Software, they executed the Batterie Aix project, where they efficiently combined experiments, systems simulations (SYS) and computational fluid dynamics (CFD) to optimize the battery thermal management, taking into consideration scalability, re-usability and future enhancements.

Using Siemens solutions to gain insight into battery usage

Electrical mobility is one of the most significant challenges in the automotive industry and one of the most important aspects is battery thermal management – the discipline that integrates several systems (cooling, batteries, etc.). It enables an optimal and safe use of the battery pack, which is imperative for racing and passenger cars. Automotive original equipment manufacturers (OEMs) must test and characterize these battery packs to safely integrate them into the car cooling system – they need to trust them. Simulation experiments play an essential role in covering a spectrum of operating conditions, such as lifetime and aging.

In the Batterie Aix project, the students needed to determine how to characterize the battery cells, optimally integrate the batteries in the vehicle to meet performance expectations and range requirements and achieve the proper design and correct dimensioning of the cooling system. The fundamental idea behind this project is to co-simulate SYS and CFD using experiments. “We needed Siemens’ solutions for battery and thermal simulations; modeling the battery and its behavior was a new field for us,” states Thomas Nyhues, student engineer, Ecurie Aix.

Executing the Batterie Aix approach

Instead of using a complex battery model to generate data, they conducted realworld experiments at the Institute for Power Electronics and Electrical Drives (ISEA) of the RWTH Aachen. By using a multi-disciplinary approach for the electrical and thermal simulation of battery packs, the team combined experiments, SYS and CFD, to provide valuable insights into battery usage. First they ran the battery testbench with different current profiles that were generated by using Simcenter™ Amesim™ software. By using the test results as input for the battery identification toolbox, the team was able to fully identify all parameters of the equivalent circuit model in Simcenter Amesim. Furthermore, this fully parameterized model can now be used for making further steps and enhancements in the project. Simcenter is part of the Siemens Xcelerator business platform of software, hardware and services.

Next was the functional mockup interface (FMI) stage, where the team exported the fully characterized battery pack as a functional mockup unit (FMU) with the FMI standard. This step enabled the exchange of temperature and heat flow between the software. Finally, they ran CFD simulations, where they imported the FMU and characterization of the pack. From here they resolved time-dependent conjugate heat transfer simulation in Simcenter STAR-CCM+™ software, using a current profile that was recorded during an endurance race. The FMI and FMU standards are supported by both solutions and enable the software to communicate with each other without any complications. This feature was implemented in the software during the project so the team was among the first to use these functions.

Additionally, the technical support provided by Siemens was a key success factor in this project. Claudio Santarelli, academic business developer at Siemens and Christopher Helbig, presales solutions consultant for system simulation at Siemens, guided Ecurie Aix throughout the project. As part of the Siemens engagement with student competition teams worldwide, the team was also provided with online learning material and documentation. Nyhues continues, “We ran into some problems with coupling the software and using it for the first time, but Siemens’ support team always offered us a solution.” The team also had access to the Siemens Support Center, which had frequently asked questions (FAQs) and tutorials the students could access at any point during the project. They could also use the Siemens learning platform, Xcelerator Academy.

Gaining realistic and trustworthy simulation models

This simulation setup can now be enhanced by design changes to optimize the battery thermal management. By using Siemens’ solutions, the team gained more realistic and trustworthy simulation models, which enable them to reduce the time and resources invested in track tests and reduce costs. “Siemens’ Simcenter STAR-CCM+ is a state-of-the-art tool for CFD simulations – especially for aerodynamic simulations,” states Nyhues. Furthermore, the simulations allow the team to access data and information that cannot be accessed with track tests. This opens new doors in the development process of the car. “It is difficult to design good test protocols that are fully characterized. By using Siemens’ software, it was fully automated – we just got the documents and then we were good to go,” states Fabian Böhm, student engineer, Ecurie Aix.

Once the battery cooling system was correctly simulated, the team could take advantage of it during the development process of the car. For example, reducing the size and the weight of the cooling system gave Ecurie Aix an advantage over other teams. “The battery identification tool in Simcenter Amesim saved us a lot of time,” continues Nyhues. “You just put your data into a toolbox and get an accurate battery model.

“The main aspect in our keys to success was the collaboration with the Siemens team,” states Nyhues. “They enabled us to get into the software and tackle any problems that arose during the project. I am still overwhelmed by the motivation of the Siemens team. It felt like they were part of our team and had the same goal.” Luca Leogrande, student engineer, Ecurie Aix, continues, “As a team, getting new members quickly up to speed is necessary to be more effective so the documentation we developed with Siemens enables us to do so.” Böhm concludes, “The collaboration with Siemens’ support team was the driving power behind the project.”

Implementing battery model in real-testing approach

The team is looking forward to using Teamcenter® software for several aspects including documentation and change management. “These two points are what makes Teamcenter interesting to us,” explains Leogrande. The team will also be implementing the battery model in the real-testing approach, such as software-in-the-loop (SiL) in combination with hardware-in-the-loop (HiL). It is anticipated future work will include using the battery model in the lap-time simulation and for a real-time controller.