Cells testing is a topic largely reproduced in research papers about Lithium-Ion batteries, with known general effects from its parameters: higher currents produce increased heat, mechanical stress and Li concentration gradients; low temperatures favour Li plating, high temperatures accelerate kinetics of parasitic reactions, and many other parameters reportedly generate specific consequences to each cell chemistry and engineering. An aspect of those tests that is surprisingly overlooked is, however, how relevant they are to real applications. Packs of electric vehicles follow irregular discharge power profiles that are not limited by fixed potentials, have optimized tab welding connections and are thermally shielded by insulating modules meanwhile count on some active cooling system. Most of them are absent in cell-level degradation studies, leading to lowly representative cycling tests.
In our work, we introduce the use of simplified synthetic drive cycles under close-to-real conditions to conduct degradation studies at cell level. Using an electric British bus average journey background, in terms of a simplified version of the UKBC drive cycle, we study how aging of commercial cells would be impacted by 4 key cycling parameters: temperature, current (indirectly in terms of power), regenerative braking and state-of-charge window (SOC). As some of them are interdependent, a reasonable comparative concept is suggested: the use of a fixed energy discharge criteria to limit the discharge of them, simulating how cells would be employed in an identical bus journey. The setup would also include electrically and thermally insulating PLA rigs, active water base cooling and tab welded connections. Cells were thermally monitored using thermocouples on different sections of its longitudinal axis.
It was observed, at early degradation, that the used cell (BAK53E) was significantly sensitive to low temperatures, with confirmed Li plating obtained in cell teardown, and sensitive secondly to absence of regenerative braking, in profiles that would lead to larger SOC windows, overcoming increased power during charge or discharge and high temperatures. We believe that such outcomes could deliver relevant information for industrials in terms of cells choice and optimized usage. As outlook, a deeper degradation mode analysis is presented, using our group’s cell analysis package PyProBE, that offers the automatized tracking of loss of lithium inventory, loss of active material and (if applicable), composite electrode composition changes during the aging.