One of the most important lifetime-limiting factors for lithium-ion cells is calendar aging. This is why many experiments are performed to characterize and model it. These experiments are typically conducted under open circuit (OC) or constant voltage (CV) condition. In OC condition, the cells are stored without any electrical connection during the rest periods, while in CV condition a constant voltage is applied by an external device like a battery tester. When designing calendar aging characterization experiments, researchers must decide which storage method to apply. In case of OC condition, it also must be defined how long the rest period between two checkups and thus state-of-charge (SOC) resetting should be. When comparing the obtained results to literature, the question arises whether results from experiments under OC condition can be compared with results from experiments under CV condition. In real-world applications, both storage methods can be found. OC condition is often present in electric vehicles, stationary battery storage systems or consumer electronics. CV condition is applied in DC intermediate circuits, e.g. in online UPS systems. In both cases, it is desired to increase battery lifetime by optimizing the degradation during rest periods.

In literature, there are only a few studies available directly comparing calendar aging in OC and CV condition. Summarizing the results of Käbitz et al, Kuntz et al and Streck et al, it can be stated that no difference in degradation has been found between OC and CV conditions for a SOC up to 90%. However, at 100% SOC, several studies show that there is a difference. Thus, the SOC region between 90% and 100% is of interest. From a certain SOC level onwards, the method seems to make a difference.
Thus, an aging study was designed which focuses on the high SOC region, comparing both storage methods directly. Three SOC levels were chosen: 100%, 96% and 92%. At each SOC level, two to three cells were tested per testing point, once under OC and once under CV condition. For the CV condition, the voltage was kept at the open circuit voltage value at the corresponding SOC level. The device under test was a cylindrical, high-energy cell manufactured by Samsung SDI (21700 50E). It has a nominal capacity of 4.9 Ah. The cathode is based on NCA, while the anode consists of graphite and silicon. The used testing equipment was a Neware battery tester (CT-4009 5V6A) and a temperature chamber from Voetsch (LabEvent T500). The temperature during the storage periods was set to 45 °C, while the checkups were performed each 35 days at 25 °C. After each checkup, the SOC was set according to the actual C20 capacity. This was repeated until a total testing duration of 455 days was reached. The checkup consists of a C1 and C20 capacity measurement, a quasi-OCV measurement in charging and discharging direction with C20 and pulses for inner resistance calculation at several SOC levels (80%, 50%, 25%, 10%). For OC condition tests, the voltage decay during storage periods was monitored by measuring the voltage weekly with a Fluke multimeter (+- 1 mV accuracy).

The obtained results show that there is no difference between OC and CV for all SOC levels for the C20 capacity development. Moreover, the differences between the SOC levels themselves independently of the storage method are rather moderate. While the capacity decreased to approximately 88% for SOC 100%, it decreased to approximately 89% for SOC 96% and to 90% for SOC 92%. For resistance increase, only at SOC 100% the inner resistance grows faster under CV conditions. For the other SOC levels, no clear difference can be found. Considering the findings from literature, it can be stated that both storage methods deliver comparable results up to 96% SOC. These findings indicate that most calendar aging experiment results should be comparable, independently of the storage method. However, the storage period duration should still be considered (see further below). At 100% SOC a stronger resistance increase for constant voltage method is observed. For resistance increase, there is a strong jump from SOC 96% to SOC 100%, in contrast to the C20 capacity decrease. Here it becomes clear that it is crucial to also consider the resistance increase to evaluate the impact of calendar aging on the cell’s state. The differential voltage analysis shows that similar degradation modes take place for OC and CV conditions, which is therefore not depicted in the poster.

The voltage measurements during OC storage periods reveal a higher voltage decay for SOC 100% compared to the other SOC levels (approximately factor three). This could be explained by the faster side reaction rate at higher voltage levels and thus faster self-discharge. The voltage decay can account for up to 75 mV during a storage period of 35 days compared to the CV voltage. This also makes clear that voltage decay should be considered when choosing the duration of the storage period. Another factor playing an important role in the explanation of the found differences is the quasi-OCV shape in the high SOC region. Due to the higher slope of the quasi-OCV close to 100% SOC, the voltage decays even faster for the same amount of discharged ampere hours compared to regions with less slope of the quasi-OCV. The results suggest using the slope of qOCV curve to find an optimal maximum SOC during operation. A trade-off must be found between choosing a higher voltage during storage and thus faster degradation, but more available capacity and a reduced maximum storage SOC, limiting the degradation speed.