© Lunghammer - TU Graz

E-batteries: The older, the safer

Studies at TU Graz show: The older an e-car battery, the lower the danger it poses. Now the researchers and industrial partners want to define parameters for the subsequent use of discarded batteries.

Within the framework of the K-Project “SafeBattery”, a team from TU Graz has been investigating the behaviour of lithium-based batteries of electric cars under crash loads for the past four years. “The performance of new battery cells is largely known, so we dealt with the entire life cycle,” explains project manager Christian Ellersdorfer from the Institute for Vehicle Safety. Together with industry partners such as AVL, Audi and Daimler, scenarios were researched that a battery can experience in the course of its life: for example, vibrations and strong accelerations caused by parking bumps, serious accidents and the constant charging and discharging of batteries.

Changes due to loading and unloading

With the help of crash tests, simulation models and calculation methods, the researchers were able to determine that vibrations and accelerations hardly influence the behaviour of batteries. However, more significant mechanical and electrical changes were seen as a result of the constant charging and discharging of the battery. Battery cells aged in this way have a higher stiffness under mechanical load. “However, the changes do not necessarily mean that batteries become more dangerous with age. On the contrary: the sum of the influences makes them safer over time because they also lose electrical energy,” says Ellersdorfer.

The investigations of Ellersdorfer et al show that cells with a strongly reduced capacity content have a weakened course of the so-called thermal runaway after an internal short circuit. The reduced energy potential of aged batteries therefore reduces the likelihood of accidental battery fires.

Benefits for the automotive industry

Thanks to the research results, manufacturers now know what they can expect from a battery cell. This enables material-saving designs and greater efficiency, as Ellersdorfer explains: “Until now, the battery was installed in such a way that deformations could be ruled out in every conceivable scenario. Now, manufacturers can make better use of installation space. And safety checks on a new cell are valid for the entire life of the battery.”

Qualification of E-batteries for a second life

In the timeline of a battery’s life, the SafeBattery consortium is now going one step further: In the recently launched COMET project SafeLIB, the changes in e-batteries are being examined even more closely together with other partners (LIT Law LAB, Infineon, Fronius, Mercedes) in order to be able to derive safety factors for subsequent use. “Used batteries with a power capacity of 80 percent are no longer suitable for electric cars, but they are suitable for stationary energy storage or for machine tools. For the first time, we are determining generally applicable measured variables in the area of safety,” Ellersdorfer describes the project.

The researchers are using the world’s only test bench technology for battery safety in the Battery Safety Center Graz, which will open at the end of 2020. There, the past life of a battery cell can be examined in an unprecedented level of detail. The legal framework for reusability (e.g. the question of liability for consequential damage) is also taken into account. In addition to the so-called “State of Health”, which reflects the existing residual capacity and performance of a battery cell, a “State of Safety” should ultimately be defined, with which the safety status of a battery can be assessed over the entire life cycle. SafeLIB has a duration of four years and ends in 2025. The Austrian Research Foundation FFG is funding the K-Project with a total of 6 million Euros (see FFG Fact sheet).

Both research projects are anchored in the Field of Expertise “Mobility & Production” at Graz University of Technology, one of five strategic focus areas of the university.