# Determination of Cycle to Cycle Battery Cell Degradation with High-Precision Measurements
## 1 Introduction  
In order to counteract anthropogenic climate change, it is important to be able to produce affordable and high quality energy storage systems for renewable energies [1, 2]. Lithium ion cells are electrochemical energy storage systems which are already widely used for this purpose, e.g., in electric cars and stationary energy storage systems, and whose development potential has not yet been exhausted [3]. In battery cell research and development, many cell parameters are of interest. Some of these parameters can be determined quickly, such as capacity, internal resistance and temperature behavior of the cell. However, the development of these parameters over time can usually only be determined with long-term measurements. Several approaches try to speed up the aging effects by exposing the cells to higher temperatures. The intention is to accelerate the parasitic reactions that degrade the cell according to [PERSON] law [4, 5, 6]. Other approaches use high cycling currents, thus making many cycles in a short time [7, 8, 9]. Furthermore, both methods can be combined to obtain even faster results [10]. However, with all accelerated test procedures, the question arises as to whether these cells still show the same behavior as in real world applications, or if they age due to excessively high temperatures and extreme loads [11, 12]. Therefore, results which examine the cell under normal operating conditions should be used in preference as shown in [13, 14, 15, 16]. The resulting problem, however, is that the duration of the cell tests now increase considerably. A study was done in [14] where cells over 4 years were observed to determine their capacity fade through calendrical and cycle aging. As shown in [16], several cells were studied over a few months during which the cells' ambient temperature as well as the state of charges (SoC) were varied in a realistic range in order to make statements about calendrical aging. In [15], self-assembled lithium cells were cycled for 600 h in order to determine calendrical aging. At the same time, in [15], a cell of the same type was stored at 100% SoC for the same period of time to determine its calendrical aging. A few new promising approaches were launched by [PERSON]'s Research Group from Dalhousie University. Most of their approaches have in common that they measure the cycling currents and voltages of the cell extremely precisely, whereby very small aging-related changes are supposed to be detected [11; 12; 17; 18; 19; 20; 21; 22; 23; 24]. In [21], it is shown that the sequence in which cells age--with the same electrode chemistry but different electrolyte additives--can be determined with high precision Coulometry, and this after only a few days of measurement. As shown in [19], even cell lifetime predictions can be made using such high-precision measurements, although it is not yet clear how reliably they work. It is worth noting that this research addresses the issue that some cells suddenly experience a severe loss of capacity due to lithium plating [25; 26]. Mostly, the researchers use self-built devices as described in [18; 22] because none were commercially available with this high measurement precision at that time. The self-built devices allow for countless analysis methods, such as capacity measurements, impedance spectroscopy, Coulombic efficiency measurement, differential capacity analysis (DCA), differential voltage analysis (DVA), delta differential capacity analysis, etc. [27; 28]. However, when presenting the results (especially with commercial systems), it is uncommon to consider that the measuring devices used could themselves have an influence on the measurement results. This means that every measured change is automatically attributed to the cell. This conclusion is often misleading as shown, e.g., for DCA and \(\Delta\)DCA in [27] because, for such precise measurements, there are considerable demands on the measurement system.