Verification of Life Estimation Model for Space Lithium-Ion Cells

Large capacity Li-ion cells with 100 Ah for satellite application had been developed in 1999, and calendar and cycle life characteristics of the cells had been evaluated under various test conditions with wide range of temperature (0 degrees C-60 degrees C), depth of discharge (3% - 80% DOD), and state of charge (0% - 100% SOC). These tests were started in 1999, and the data have been accumulated until 2008 for around ten years. From these results, we have confirmed that GYT Space Li-ion cells have sufficient capability to achieve the mission life requirements for several kinds of artificial satellites. Furthermore, we have discovered that our simple life estimation model needs to be modified to consider SET growth blocking mechanism. It means that the SET growth is blocked by the adjacent SET layers, therefore calendar capacity loss is affected by not only its test term, temperature, and state of charge but also its calendar capacity loss values. Our modified estimation formula is that a rate of calendar capacity loss is decreased in proportion to the 2.4(th) power of the calendar capacity retention. By using the modified formula, the estimation results show very good fitting with the long term cell test data for ten years.