# Extending Battery Lifetime by Avoiding High SOC
## 6 Discussion  
The lifetime tests on this cell showed, contradicting previous studies on LFP [1] and NMC cells [3], that the low SOC levels is less detrimental compared to higher SOC levels. Additionally, even higher C-rates show less ageing compared to the tests conducted in higher SOC levels. An important factor that needs to be pointed out is the definition of the SOC window. [PERSON] et al. [3] defined the SOC window from 2.5-4.2 V, compared to the 2.8-4.15 V used for the here studied cells. If we consider the corresponding voltage level for the SOC intervals instead, the results produced by [PERSON] [3] then prove to be similar to the ones presented in this study. From this, we can conclude that how we define the SOC window is important since the ageing is strongly linked to the SOC level, i.e. the voltage level of the cell. Even though the impact of high SOC on ageing is a well known fact, few studies have focused on the impact of ageing when not using the full battery capacity. The costumers wishing for a longer driving range of EVs have controlled the research so far. As shown in the _Swedish car movement data project_[9], the most common trips are 30-50 km, and today the driving range of the EVs are longer than this, opening up the field for updated battery management strategies and, with this, the possibility of prolonging the battery lifetime. The synthetic cases studied in Figure 8 show how the C-rate, DOD, and SOC level impact the ageing. The fastest ageing is achieved for S8, 1C 40-90%, while the same DOD, though in 0-50%, S7,  
Figure 11: Resulting capacity degradation when including calendar ageing in 25 \({}^{\circ}\)C for user case 1 at 15%, case 1 reference at 90% SOC and a combination of 15% and 90%. manages more than double the number of FCE until reaching 80% capacity retention. Other cases showing faster ageing are S6, only using 20% DOD in 70-90% SOC, and all three C-rates for S3, 10% DOD in 80-90% SOC. These results indicates that the SOC level is more important than the C-rate. The S1, S2 and S3 cases show even more clearly that the impact of SOC level is higher than the impact of C-rate. Important to point out is that, as the SOC level is increased, the impact of C-rate also seems to increase, S3 using 2C shows similar ageing as S8 though only using 10% DOD. In S1-3, the model was extrapolated outside the available measured data, in order to estimate the ageing in 0.5C. Even if the trend seems fairly straightforward, the results for C-rates lower than 1C have to be treated with prudence. For vehicle applications, the type of driving is very important for the ageing. Higher speeds require substantially higher C-rates, and the losses for the air drag grows with the square of the vehicle speed. If the battery is small, as for Vehicle 3, the required C-rate will be high, resulting in faster ageing. For the three vehicles studied in this work, driving the distance of 25 km on highway used 30, 26 and 62% DOD, respectively. When using mixed driving, only 21, 19, and 46% DOD were required to complete the trip. The highway driving required higher C-rate compared to the mixed driving, resulting in faster ageing. For all three vehicles, the choice to charge only up to 50% SOC highly improved the lifetime, for Vehicle 3 with 1.4 years. In the mixed driving case, the extension of the lifetime was even higher. Including calendar ageing to the model showed that the lifetime could be reduced with more than 60% compared to the case only considering cycling ageing. This contribution could be reduced to 35% lifetime reduction by planning the charging, thus maintaining a low SOC level during parking. Intuitively, the importance of calendar ageing is larger when the cycling ageing is moderate and the expected lifetime long. When the cycling ageing is rapid, the calendar ageing is less influential. For the most aggressive drive case, Vehicle 3 in WLTC Extra High, the lifetime reduction was as low as 13%, assuming storage of the battery at 15% SOC.