State of charge and heat impact on Li-ion batteries during calendar aging

Abstract: Rechargeable batteries that are powering our everyday electronics go through their lifetime with countless charge/discharge cycles, after a few years the capacity has dropped noticeably. But how about batteries that are working as a back-up power system for when connection to the AC mains is not possible, they might only perform a handful of cycles but does this mean they last forever? On behalf of Getinge AB, this bachelor thesis aims to explore the subject named as ‘calendar aging’. Focus of the project has been mainly the impact of charging capacity, State of charge, on battery life for calendar aging and secondly the impact of heat on battery. Elevated environmental temperature during calendar aging test would imply acceleration in battery aging. The study is made with partly literature study and partly experimental testing on batteries during relatively limited time, though the results could be benchmarked with likely studies by others. The test was to investigate 12 units of NMC Li-ion batteries charged to 3 different levels of State of charge, 90 %, 95 % and 100 %, where 2 from each State of charge level were in a controlled heated environment and the other 2 were at room temperature. The test was in progress for 8 weeks and every two weeks measurement and data logging were collected and, if required, the batteries were recharged to origin voltage level. All data was documented in  and analyzed in Excel.  The results acquired were not significant enough to draw any conclusions but did show some tendency as would be expected from the literature studied during the time of the project. Future work is recommended to continue the test in this project to acquire more useful results. Further improvements in testing equipment should also be considered.