batteries : E-batteries: The older, the safer

As part of the "SafeBattery" K-project, a team from TU Graz has spent the past four years studying 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, research was conducted into 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.

>> Also read our story about lithium-ion batteries and the dangers they pose <<

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," Ellersdorfer said.

The studies by Ellersdorfer et al show that cells with greatly 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 thus reduces the probability of accidental battery fires.

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."

In the timeline of a battery's life, the SafeBattery consortium is now going one step further: in the recently launched SafeLIB project, the changes in e-batteries are being examined in even greater detail together with other partners 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 world's only test bench technology for battery safety at 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 conditions for reusability are 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" will ultimately be defined, with which the safety status of a battery can be assessed over the entire life cycle.

>> Also read our story about lithium-ion batteries and the dangers they pose <<