Using DC/DC-Converters to Investigate the Ageing of Lithium-Ion-Batteries Under the Influence of High Frequency Current Ripple

摘要

Over the past few years, the e-mobihty's demands on battery systems have been increasing steadily holding complex challenges regarding prediction of battery states and the overall system's reliability. Moreover, nowadays, every battery system comes with a converter that is based on fast switching, power electronics that induce high frequency noise and current ripple on the battery. Therefore, standard ageing test systems, that only cover cycle depth, mean state of charge or temperature, need to be complemented by additional test systems and the respective investigation of the influence of power electronics, their periodic switching pattern and the resulting ripple current. Being aware of the influence of the whole system on the battery's lifetime may lead to improving the operating schedule and optimising the design of the whole power unit. To conduct those aforementioned investigations, a novel battery test circuit, which is based on a half bridge dc/dc-converter has been proposed. This circuit imposes a triangular current ripple on the dc-part of the current and is used to conduct cyclic ageing with different cycle depths on a commercial and cylindrical (18650) Li-Ion battery under a controlled environment with a constant temperature. On a regular basis, the cells' parameters are measured and compared to cells that have been aged with commercially available cell test systems for validation. In addition to direct influences of current ripple on battery ageing, the cumulative and premature breakdown of ripple aged cells or their internal contact loss, respectively stands out. The authors believe that it is connected to inhomogeneous current distribution and therefore locally increased losses. Apparently, the battery system as a whole is more demanding than it is possible to analyse with conventional ageing methods and systems so that highly flexible, extensively usable battery test systems will have their noteworthy share in the field of battery diagnostics, thus leading to saver and more reliable battery systems.