# Determination of Cycle to Cycle Battery Cell Degradation with High-Precision Measurements
## 3 Experiments
### Differentiating between Cycle and Calendrical Aging  
In order to separate cycle aging and calendrical aging from each other, cells are cycled at different current rates. It is expected that differences between the capacity degradation rates will be detected, which will indicate the respective cycle aging and calendrical aging. For this purpose, the three measurements C/4, C/12 and C/36 are used.  
If the calendrical aging of the cells would be the dominant aging mechanism and the cycle aging is negligible, the capacity degradation curves plotted over time (Figure 11) should overlap. However, if cycle aging is dominant, then the capacity degradation curves plotted over the number of cycles (Figure 12) would overlap. To all appearances, the second case is present, so the cell is damaged mainly by cycle aging. This means that calendrical aging is negligible in the C/x-scheme because the SoC in this scheme is an average of 50%, which is too low to cause significant calendrical aging in this cell type.  
The following observation further proves that cycle aging is dominant: At the same number of cycles, the cells with a higher charge rate age slightly faster which can be seen from the capacity decrease as shown in Figure 12. This is counterintuitive because, at the same number of cycles, cells with a higher charge rate were in fact exposed to cycling for a shorter measurement period and therefore have less calendrical aging, but still the capacity decrease is faster for a higher cycling current--hence having a shorter measurement time--and the same number of cycles.  
This supports the claim that calendrical aging hardly occurs, and cycle aging is the dominant degradation mechanism for this cell type cycled with C/x-schemes.  
Figure 11: Normalized capacity decrease observed over the measurement time. For this purpose, an LG INR18650 MJ1 cell was cycled with C/4, C/12 and C/36 at 40 °C ambient temperature.  
By investigating Figure 12, one could presume that the aging of the cell increases over-proportionally with the charging current because the C/4 measurement shows a faster decay than the C/12 and C/36 measurements. To investigate this presumption in more detail, the difference between consecutive capacitance measurements was calculated for different C-rates as shown in Figure 13. As it turns out for all measurements, they have similar aging rates as shown in Figure 13. Initially, the rates of the C/12 and C/36 measurements differ slightly from the C/4 measurement, but they gradually begin to converge until, after 13 cycles, the C/12-curve finally overlaps. It can be stated that the cells cycled with the C/x-scheme mainly experience cycle aging and almost no calendrical aging. Furthermore, the cycle aging rate has hardly any current-dependence for the chosen magnitude of cycling currents. Therefore, the measurement results from Figure 13 already turn out to be the sought cycle aging rate. It is assumed that the cycle aging rate is the same for the High-SoC scheme, as the cell is cycled in exactly the same way as in the C/4 measurement.  
What turns out to be surprising is that the aging rate of the cells with respect to its capacity decreases significantly in the course of the first 80 cycles (for C/4, the rate changes by factor 5). This transient shows that it does not make sense to look for a single value regarding the cycle aging rate, but that only a curve for the cycle aging rate comprehensively describes the actual process. This transient process can be well explained with the buildup of the solid electrolyte interface as shown in [32].  
Figure 12: Normalized capacity decrease observed over the number of cycles. For this purpose, an LG INR18650 MJ1 cell was cycled with C/4, C/12 and C/36 at 40 \({}^{\circ}\)C ambient temperature.