TU Graz puts car batteries to the test

Batteries in electric vehicles have a limited service life; They age with each new charge: this effect means that they store less and less energy, so their range and performance decrease. Things currently became critical at around 160,000 km. After that, car batteries could certainly find other uses – for example in stationary household storage systems, where they are no longer under as much pressure as in a car – for example for temporarily storing excess energy from solar and wind power plants.

Discoveries thanks to comparison

Whether batteries can continue to be used reasonably depends primarily on their remaining performance. Researchers at the Institute of Vehicle Safety at Graz University of Technology have collected a whole set of indicators that can be used to assess the condition of discarded lithium-ion cells.

The researchers compared discarded lithium-ion cells with similar, new cells in the laboratory: they recorded 31 different measurement values ​​during charging and discharging cycles. 13 of these indicators turned out to be meaningful. The Graz experts included the charging and discharging capacity, the temperature difference between the electrodes during the charging process and the relaxation behavior of the battery cell after charging.

Aging can be identified

“Using these indicators, we can draw conclusions about the aging state of lithium-ion batteries and draw preliminary conclusions about different usage profiles without having to rely on important data protection information about the usage history of the batteries,” explained Jörg Moser, president of the company. From the Graz Battery Safety Center at the Vehicle Safety Institute. On this basis, one should be able to determine whether the battery is in principle suitable for continuous use in a particular field of application.

Safety and economic efficiency

If the batteries are to be used again, their safety condition is also important, as chemical changes occur in the materials over the years. “It is important to understand battery cells and the processes, interactions and changes that occur in them in detail so that we can qualify them regarding their safety behavior,” said Christian Ellersdorfer. He is the leader of the COMET SafeLIB project at the Vehicle Safety Institute, where several research institutions as well as automotive and technology companies are working on the safety evaluation of new and used lithium-ion batteries. The first results should be available at the end of the year.

According to the assessment of the researchers in Graz, it will take a few years before used batteries are widely used in subsequent applications, making electric mobility more sustainable over the entire life cycle. In addition to the safety of different battery technologies, the economic feasibility of second life applications and legal issues around data protection, warranty or liability must also be taken into account. “This results in an interdisciplinary research area on which we at Graz University of Technology would like to work together with national and international partners in further research projects,” Ellersdorfer concluded.