E-car batteries

The older, the safer

6. Mai 2021, 9:00 Uhr | Irina Hübner
Das Verhalten neuer Batteiezellen ist weitgehend bekannt, doch ein Forscherteam an der TU Graz beschäftigt sich mit dem gesamten Lebenszyklus.
Studies by Graz University of Technology show: Batteries in electric cars become safer with age. In the picture: Project manager Christian Ellersdorfer with colleague Christian Trummer at the battery crash facility of the Battery Safety Center Graz.
© Lunghammer – TU Graz

Studies at Graz University of Technology show: The older an e-car battery is, the lower the danger it poses. For the automotive industry, this could mean advantages when installing the battery.

As part of the SafeBattery K-project, a team from TU Graz has spent the past four years investigating the behavior of lithium-based batteries in electric cars under crash loads. »The performance of new battery cells is largely known, so we dealt with the entire life cycle,« explains project leader Christian Ellersdorfer from the Institute for Vehicle Safety. Together with industry partners such as AVL, Audi and Daimler, the research team looked at scenarios that a battery can experience over 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 charging and discharging

With the help of crash tests, simulation models and calculation methods, the researchers were able to determine that vibrations and accelerations hardly affect the behavior of batteries. More significant mechanical and electrical changes, however, were seen as a result of the constant charging and discharging of the battery. Battery cells aged in this way have greater stiffness under mechanical stress. »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 studies at Graz University of Technology show that cells with a greatly reduced capacity content have a weakened course of the so-called thermal runaway after an internal short circuit. The lower energy potential of aged batteries thus reduces the probability 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 of a new cell have validity for the entire life of the battery.«

Qualifying 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 to be illuminated in even greater detail 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 e-cars, but they are very suitable for stationary energy storage or for machine tools. For the first time, we are determining generally applicable measurement parameters in the area of safety," says Ellersdorfer, describing the project.

The researchers are using the test bench technology for battery safety at the Battery Safety Center Graz, which will open at the end of 2020. There, the pre-life of a battery cell can be examined in a particularly high degree of detail. The legal framework for reusability is also taken into account, for example the question of liability in the event of consequential damage. 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 is thus to be defined, with which the safety status of a battery can be assessed over its entire life cycle. SafeLIB will run for four years and end in 2025. The Austrian Research Foundation FFG is funding the K-Project with a total of 6 million euros.

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