# A Strategic Pathway from Cell to Pack-Level Battery Lifetime Model Development
## 3 Degradation Results  
The crucial impact factors that are employed in the cycle and calendar life campaign, had a diverse contribution to total aging. The internal resistance deviation for the studied cells is not analyzed, thoroughly as very slow growth is observed. Thus, the lifetime results are presented in terms of capacity decay in the work. In the following, the impact of the influential parameters is analyzed for the capacity degradation.  
The cycling capacity retention is stress factor-dependent among which the DoD and temperature play a major role. The DoD or cycling SoC range may affect battery life heavily at different temperatures along with other operating conditions. The impact of deep and shallow cycling on battery life is observed for the LTO cell which is common for other lithium technologies as well [40; 41; 42]. The DoD dependency at 45 \({}^{\circ}\)C where higher DoD cycling tends to reach the EoL criterion sooner than lower DoD cycling. Although the shallow cycling measurement does not include the full life until 80% SoH, the already existing data reflects the DoD dependency showing the irreversible effect of full DoD cycling. It has been found that the cycle life of this battery cell can be doubled theoretically if cycled at 80% DoD compared to 100% DoD. Further lower DoD cycling can extend lifetime as well.  
Figure 1 explicitly exhibits dependency for the cells that are cycled at different conditions. When cycled with full DoD range continuously, the higher temperature dependency is significant and clear. The prominent impact of high temperature such as 45 \({}^{\circ}\)C cycling degrades the cells to EoL after 7500 FEC, while the room temperature cycling loses 6% of the capacity after 6400 FEC. The shallow 20% DoD (in the 50% mid-SoC region) degrades much slower than full DoD cycling (Figure 1d) indicating that its usage in a hybrid application or urban driving may make it even last long. The displayed figure also reports a consistent temperature effect in the studied DoD cycling results where it is found that the temperature is inversely proportional to the capacity fade. This means that the studied cell type aging is prone to the lower temperature as shown in Figure 1b for 80% DoD cycling. At this condition, the cells on average lost 7% of the capacity after 7500 FEC at 45 \({}^{\circ}\)C, while the 0 \({}^{\circ}\)C cycling condition experienced 11% capacity loss after only 2000 FEC. The temperature effect is found to be the key degradation stress factor that would drive the model parameterization.  
The impact of mid-SoC is not found at available lower DoD cycling conditions; however, cycling at 20% mid-SoC (when DoD is 40%) seems to have the least capacity fade as shown in Figure 1c, but conclusive remarks could only be made with a complete first-life dataset. The minor stress is still considered to be included in the model parametrization to have an integrated impact on this condition.  
The charge-discharge cycling rates can significantly contribute to Li-ion batteries capacity degradation if not power-optimized. The charge rates are studied at room temperature and for an 80% DoD cycling scenario. The aim was to obtain the current rate influence; however, the degradation trend is found to be unclear which is a challenge for the modeling work. Due to ambiguous characteristics and lack of discharge variation data, the C-rate impact is neglected in the model parameterization.  
Moreover, the calendar life tests as per the designed test matrix have generated trivial information. After 18 months of storage tests, the cells have showcased a robust behavior (up to 2% negative fade) with little or no degradation sign. Thus, the calendar life impact is excluded from the modeling framework due to its negligible contribution over the lifetime. The extremely long life of LTO anode-based cells is acknowledged in the literature as well and in line with the manufacturer's claim [43; 44].