© 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.