# Extending Battery Lifetime by Avoiding High SOC
## 5 Results
### Cycling and Calendar Ageing  
Until now, only cycling ageing has been considered. The next step is to add calendar ageing. The cycling and calendar ageing were added together by the number of FCE driven each day and the percentage of hours in storage per day, \(time_{cal}\), according to  
\[Cap_{case1}(Cycling,Calendar)=Cap(FCE/day)+time_{cal}\cdot Cap_{cal}(SOC,Days,25 \,^{\circ}C). \tag{12}\]  
As can be seen in Figure 3, the temperature highly affects the ageing. The average temperature during a day is highly dependent on the location in the world and season; here, an average temperature of 25 \({}^{\circ}\)C has been used. From (5) and (6), the lifetime, until reaching 80% capacity retention, when only considering calender ageing, is 10 and 32 years, for 15 and 90%, respectively. According to this, storing the battery at 15% SOC instead of 90% SOC will triple the lifetime. The calendar ageing combined with driving was studied in the WLTC and WLTC Extra High drive cycle case for Vehicles 1 and 3. It is assumed that a full charge of the vehicle battery takes 8 h and the vehicle is fully charged five times a week, i.e., 40 h/week. The driving time differs, depending on cycle and vehicle, between 2-12 h/week. This results in 69-75% calendar ageing time each day. For a simpler comparison the same calendar time, 72%, was used in the following. In Table 7, the contribution from calendar ageing are presented as percent reduced lifetime compared to the pure cycle ageing. For Vehicle 1 driven according to WLTC, only a low C-rate is required and the cycling ageing is very moderate, resulting in an approximate lifetime of 16 years at 80% capacity retention. Including calendar ageing at 90% SOC, the lifetime is reduced to just above six years, a 61% lifetime reduction. Storing the battery in 15% SOC results in a lifetime of 11 years, a 35% lifetime reduction. For Vehicle 3, the lifetime reduction when including the calendar ageing for the WLTC drive case at 90% SOC is 55%, and only 29% at 15% SOC. For the WLTC Extra High, the C-rates required are higher resulting in a shorter cycle lifetime. For Vehicle 3, the cycle lifetime is approximately 5 years and combining the calendar ageing reduces it to four and three years for 15% and 90% SOC, respectively. In this case, the contribution from the calendar ageing is much smaller, only reducing the lifetime with 13% and 32%. The corresponding values for Vehicle 1 is a reduction of 21 and 46%, respectively. Now, considering one of the user cases, Case 1, where the vehicle is only used for 5 h/week and charged during 42.7 h/week, resulting in the vehicle battery being in storage 72% of the time. In Figure 11, the results from Case 1 and Case 1 ref is presented, and three calendar ageing examples are shown. In the first example, it is assumed that the battery is used in accordance with Case 1, stored at 15% SOC and that the charging is planned to be finished just before usage. In the second example, Case 1 reference, the battery is charged as soon as the driving is finished and then stored fully charged, i.e., 90% SOC. In the third example, a mixed charge behaviour is assumed. In the reference case, the cell is only discharged to 60% SOC during weekdays and the battery is therefore assumed to be charged to 90% after each drive, i.e., storing the battery at 90% SOC 5 days a week. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Case** & **Calendar** & **Cycling and** & **@15\% SOC** & **@90\% SOC** & **57 Days @90\% and** \\  & **Time (\%)** & **Charging Time (\%)** & **(\%)** & **(\%)** & **2/7 Days @15\% SOC (\%)** \\ \hline WLTC, Vehicle 1 & 72 & 28 & 35 & 61 & 56 \\ WLTC, Vehicle 3 & 72 & 28 & 29 & 55 & 49 \\ WLTC Extra High, Vehicle 1 & 72 & 28 & 21 & 46 & 40 \\ WLTC Extra High, Vehicle 3 & 72 & 28 & 13 & 32 & 27 \\ Case 1 & 72 & 28 & 37 & (63) & (58) \\ Case 1 ref & 72 & 28 & (27) & 52 & 47 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Ageing reduction when adding calendar ageing at two different SOC levels to the cycling ageing for six different cases.