Experimental Lifetime Investigation of Series Connected Li-ion Cells at Different Operation Strategies

Battery systems consisting of serially connected cells suffer from uneven cell degradation. This phenomenon can be observed, even if li-ion cells of the same type and batch are cycled in a fixed voltage window at the same temperature. The uneven cell degradation has a significant impact on the usable capacity of a battery system as the system capacity is limited by the capacity of the weakest cell. This issue reduces the battery lifetime and increases the costs. In a former work, li-ion cells were cycled individually and the different cell degradation could be observed significantly. In this work, aging tests are conducted with cells in series connected conditions to investigate the uneven cell degradation behavior. Cyclization of series connected cells could lead to a higher spread of cell degradation as weaker cells are deeper cycled.
Three aging tests, each consisting of eight li-ion cells are conducted in this work. The eight cells of each test are cycled with the same current to investigate the aging behavior under comparable conditions as in a series connection. The first test is conducted to investigate the aging behavior of a conventional system with series connected cells and passive balancing. In the second and third test, the uneven cell degradation is actively compensated with the operation strategy of the balancing circuit. In the second test the voltage window are changed with respect to the cell’s state of health. In the third test the cell current of each cell is changed with respect to the cell’s state of health.
Preliminary results show that cells in the first test suffer a bigger capacity diversion over time than cells in the second and third test. Such operation strategies could be implemented in real systems to reduce the different cell degradation and to prolong the systems lifetime. Further analysis and conclusions will be given on the conference poster.