Is a lithium ion battery still the same as new one after long time storage? With this question, let’s “reveal the mask” of battery calendar life.

1.    What is battery calendar life

Battery’s life usually refers to cycle life and calendar life (storage life). Among them, the cycle life refers to the time required for the battery to reach the end of life in working cycles or normal cycles while the calendar life is defined as: the time required for the battery to reach the end of life at a certain reference temperature, open state, ie the life of the battery in standby mode. In summary, calendar life is an assessment of the time effect on battery’s performance under conditions that minimize battery use.

2.    Overview

Studies show that for a battery calendar life, it’s mainly related to the below factors:
* Battery capacity via storage time
* Battery capacity via storage temperature
* Battery capacity via SOC
* Battery impedance via storage time

2.1 Battery capacity via storage time

2.2 Battery capacity via temperature
It’s found after testing: a lithium ion battery with a 50% SOC, the capacity degrading is slightly higher at 20°C than 0°C. The capacity decay rate at 45°C is twice that of 20°C. Similar attenuation trends observed at 100% SOC are similar to trends at 45°C. See below Figure 1:

                                                                    Figure 1

2.3 Battery capacity via SOC
The 100% SOC cell capacity decay rate is significantly higher than the 50% SOC cell decay rate, which is the case at any temperature, as shown in Figure 1. Many studies have found that high storage SOC accelerates capacity fade. Studies have shown that the low graphite anode potential at high SOC contributes to electrolyte reduction and SEI growth, and thus leads to accelerated lithium loss during calendar aging. At 25% SOC and 75% SOC at 20°C does not obey this trend. The 25% SOC cell has a faster decline in cell capacity than 50% SOC storage. The cells stored in 75% SOC are the fastest in capacity degradation in all SOC-level storage cells.

2.4 Battery impedance via storage time
The battery impedance was studied by current pulse measurement and EIS. Current pulses can determine the overall cell resistance at a certain SOC, temperature, and current level. The EIS is performed only at open circuit voltage (OCV), but it describes the change in the electrochemical phenomenon inside the battery in more detail than the pulse test. See Figure 2. 


                                                                      Figure 2

Summery

1.     A newly produced battery after long storage, the performance is degrading. With the time passing by, its capacity suffers loss, voltage gets down while impedance gets increased. When the aging reaches a certain degree, the battery is considered to reach its end of life. 

2.   
If a battery needs to be stored for a long time without any use, the battery should be separately stored from the device. And the storage condition: with 50% SOC at temperature 0~20°C is kindly recommended. Every 3~6 months, it’s required to cycle (charge & discharge) the battery for 1~3 times to keep the battery’s inside materials active.