**1. Introduction**

Battery energy systems for stationary storages as well as for electric vehicles that are based on lithium-ion batteries are widely regarded as one of the key technologies to realize the energy transition [1]. A determined shift could mitigate the most severe aspects of climate change so that in recent years the research on battery materials, modelling, ageing, systems and integration has been thriving [2]. Yet, amongs<sup>t</sup> others, a lot of open questions remain in the field of diagnosing batteries connected to power electronics in practical conversion systems although they are already comprehensively used in stationary systems and electric vehicles alike. Batteries, connected to power electronics that play a vital role in stabilising voltage quality and power delivery have been considered for many years as an important part of so called microgrids that could be a method to secure the energy supply in power grids with highly distributed feed-ins [3–5]. Moreover, as electric vehicles will have spread extensively, their battery packs are considered to be used to buffer peaks in power demand or dips in power supply [6] in vehicle to grid systems thus demanding sophisticated charging and discharging systems that are based on various power electronic circuits [7–9]. In summary, every battery in battery energy systems is in some way connected to power electronics, especially DC/DC-converters. They are based on controlling semiconducting, transistor-based high frequency switches to modulate the desired voltage or current respectively. Therefore, if costly and bulky filtering measures were not installed, the cells would be under severe high frequency switching stress. High frequency current waves, called 'current ripple', would be induced on the batteries and could lead to battery ageing mechanisms that are still unknown or render known ageing mechanisms more severe. If the results showed no further influence and therefore implied that current ripple does not shorten the battery life, the results could still lead to improvements in the design of battery energy systems as filters that are normally used to even out ripple, e.g., in [9–11], could be reduced in size or even be omitted. It is expected that the severity of current ripple rises with future converters that are based on silicon carbide (SiC) that operate at very high frequencies of several tens or even hundreds of kilohertz [12]. However, the common usage of switching frequencies considerably higher than a few kilohertz are not expected to excite electrochemical reactions thus high frequency current ripple is not expected to have any severe impact on battery ageing just as it is suggested by the following overview.

In the last years, a small amount of research groups have dealt with those particular questions in various ways: In [13], De Breucker at al. investigate the influence of current ripple, induced by a DC/DC-converter connected to a high voltage battery pack that could occur in a plug-in hybrid vehicle. Based on the switching frequency of 8 kHz the double layer is supposed to be mainly influenced by the ripple. Instead, their results show that the temperature plays a much more important role. It should be noted, that only two battery packs have been investigated. In comparison, the authors of [14] superimpose sinusoidal current waves with different frequencies up to 14.8 kHz on the DC current that charges and discharges 15 different 18,650 lithium-ion cells with a nickel cobalt aluminium oxide positive electrode (NCA) in a more detailed ageing test. It is argued that the aforementioned heat generation, leading to faster battery ageing, might also be caused by the imposed ripple current. Yet, a direct formula or factor is not given. A second study that deals with the effects of superimposed sinusoidal current waves can be found in [15]. Instead of NCA, more than 18 commercial cells with a positive electrode that is made of nickel manganese cobalt oxide (NMC) are used. Again, the authors stress that elevated temperatures cover possible effects originating from small current ripple. Finally, Bessman et al. give a short but comprehensive review of additional research in this particular field in [16]. Moreover, they carry out measurements with underlying triangular waves using twelve larger prismatic cells that are more comparable to cells that are used in high energy and/or high power battery storage systems. Compared to the other studies and most of those further mentioned within them, the work that is presented in [14] stands out since it is one of the very few that suggests a significant connection between current ripple and accelerated ageing. Moreover, most explanations remain vague and have to be categorised only as well thought-out assumptions. Thus, this present study has two goals: On the one hand, it is a further investigation on the effects of current ripple induced by practical DC/DC-converters on cylindrical 18,650 NMC lithium-ion batteries. The results of comparative cyclic ageing tests using ripple currents and comparatively unperturbed DC-current respectively, are shown and analysed thoroughly. On the other hand, the authors try to give further explanations as to why it turns out to be quite challenging to find reliable answers to the specific question of how exactly alternating currents affect battery ageing. The particular approach at hand is organised as follows: Section 2 is a detailed description about the hardware setup, the design of the cyclic ageing tests and the investigated battery are introduced. For the ageing tests, a DC/DC-converter to cycle the batteries has been developed. Its working principle and control algorithms are introduced shortly. Furthermore, regarding the test procedure, the converter is accompanied by conventional battery test equipment, that delivers mostly undistorted DC-currents, so that correspondent ageing tests with DC-currents that do not have significant current ripple can be carried out. Those ageing tests follow a set of rules that are established to be able to analyse the preassigned ageing parameters, to make the ageing tests as comparable as possible and to narrow down side effects that could overlay the influence of current ripple. In Section 3 the ageing results for a representative batch of ageing parameters is given, described in detail and more uncommon methods are introduced shortly. The depiction of the ageing results is focused on the comparison between the conventionally aged cells and those aged with ripple current. Subsequently, the results are discussed and analysed critically at the ends of each subsection. To conclude, in Section 4 the results are classified and rated. Based on this, further advice

for the practical use of batteries that are connected to power electronics is given so that future battery energy storage systems can be used in an optimal way.
