Project of the TU Graz

2nd life for car batteries as stationary storage units

Electric car batteries age with every charge: This effect means that they store less and less energy, which reduces their range and performance.

(Bild: thinkstockphotos.de)

Batteries in electric cars have a limited service life because their capacity steadily decreases over the years. The decommissioned e-car batteries can already be recycled today. Before that, however, they could also be used as stationary power storage units. However, the specific ageing condition would have to be known for this. Researchers have now collected a dozen condition indicators, as reported by TU Graz.

Batteries in electric cars age with every new charge: This effect means that they store less and less energy, which reduces their range and performance. It currently becomes critical at around 160,000 kilometers. After that, the car batteries could well find further uses - for example in stationary home storage systems, where they are no longer subjected to as much strain as in cars - such as for the temporary storage of surplus energy from solar and wind power plants.

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

13 indicators for battery condition identified
The researchers compared discarded lithium-ion cells with identical, new ones in the laboratory: During the charging and discharging cycles carried out, they recorded 31 different measured values. 13 of these indicators proved to be meaningful. These include the charge and discharge capacity, the temperature difference between the poles during the charging process and the relaxation behavior of the battery cell after the charging process.

Using these indicators, we can draw conclusions about the ageing status of lithium-ion batteries and draw initial conclusions about different usage profiles without having to rely on data protection-critical information about the usage history of the batteries.

Jörg Moser, Leiter des Battery Safety Center Graz

"Using these indicators, we can draw conclusions about the ageing status of lithium-ion batteries and draw initial conclusions about different usage profiles without having to rely on data protection-critical information about the usage history of the batteries," explained Jörg Moser, Head of the Battery Safety Center Graz at the Institute for Vehicle Safety. On this basis, it should be possible to decide whether a battery can in principle be considered for further use in a specific area of application.

The safety factor is also relevant
However, the safety condition of the batteries is also important for their reuse, as chemical changes occur in the materials over the years. "It is crucial to understand battery cells and the processes, reactions and changes that take place in them in detail in order to be able to qualify them in terms of their safety behavior," said Christian Ellersdorfer from the Institute for Vehicle Safety, where several research institutions as well as automotive and technology companies are working on the safety-related assessment of new and used lithium-ion batteries. The first results should be available by the end of the year.

According to the Graz researchers, it will be several years before used batteries are widely used in subsequent applications, making electromobility even more sustainable over its entire life cycle. In addition to the safety of different battery technologies, the economic viability of second-life applications and legal issues relating to data protection, warranties and liability must also be taken into consideration. "This results in an interdisciplinary field of research that we at TU Graz want to work on together with national and international partners in further research projects," says Ellersdorfer.