**4. Conclusions**

In this article, the influence of practical DC/DC-converter induced high frequency current ripple on the ageing of conventional 18650 lithium-ion batteries is investigated. After numerous ageing tests, the measurements strongly indicate that there is no significant effect of half bridge converter induced current ripple on battery ageing that outmatches the influence of a deep *DOD*. However, by reducing the influence of the cycle depth by using Δ*DOD*2 = 10 %, slight deviations between conventionally aged and rippled cells are visible, regardless of the measurement as seen in Figures 4b, 6c and 7b,d. Yet, a distinct relation between current ripple and these deviations remains debatable. These statements incorporate themselves into the literature and support the findings of [16] and the literature overview within. In addition to further literature, e.g., [13–15] the research at hand is able to provide some more

attempts to explain whether current ripple could be harmful in general and why the detection of any current ripple affected ageing parameters is rather difficult.

Investigating the influence of current ripple or, more generally speaking, the influence of AC-excitation on battery ageing implies cyclic ageing tests that most likely influence battery ageing more than alternating currents as long as the ageing test is made from a practical perspective. The amplitude of the current should stay within the battery specifications and the current profile itself is orientated to reproduce typical practical dimensions, i.e., limited current ripple. In concordance with [14,16], it is expected that alternating currents with amplitudes beyond battery specifications or at least designed in a way so that the effective value is drastically higher than the mean value lead to elevated heating of the batteries. However, overextending this approach would lead to accelerated ageing because of higher temperatures resulting from the high rms-value of the ripple currents. For comparison: In this study, the temperatures of the rippled cells are elevated by not more than 2 K compared to the conventionally cycled cells. It is expected that ageing tests that overextend on the deviation of the effective value from the mean value of the ripple current would reveal that the battery ageing would be influenced much more by the presence of considerably elevated temperatures than the influence of ripple currents. On top of that, the influence of elevated temperatures has already been discussed satisfactorily in the literature, e.g., in [21]. Moreover, such studies would be comparable to the result of this study that a large *DOD* affects ageing much more than current ripple. In general, indicated by the slight deviations between the cells cycled with Δ*DOD*2 = 10 %, assumed influences of current ripple are only visible if other ageing influences are minimised.
