Online aging determination in lithium-ion battery module with forced temperature gradient

Voltage imbalance in lithium-ion battery packs, which leads to impaired utilization of the whole energy-storage system, is often linked to different self-discharge rates. Despite the established use of balancing circuits, neither the true origin of voltage imbalance nor the benefits of cell balancing are as yet completely understood. In this study, a forced temperature gradient along six in-series connected, commercial 18650 nickel-rich/SiC cells was applied during cycles resulting in cell temperatures of between 25 ∘C and 30 ∘C. Every 20 cycles, the 6s1p module was equalized using dissipative balancing. Aging behavior was analyzed using checkup measurements, differential voltage analysis (DVA) and conclusive scanning electron microscope (SEM) imaging of negative electrodes. The results obtained in this work reveal that the forced temperature gradient caused different degradation rates, whereas the colder cells exhibited aggravated aging behavior, which was linked to lithium plating. Furthermore, the lithium plating caused the majority of the voltage drift within the module. The application of dissipative balancing enabled an improved utilization of the module, and increased the discharge energy. Finally, the cumulative balancing charge reflected the capacity differences between the cells, and could therefore be used for the online determination of relative aging of single cells or cell blocks in lithium-ion battery packs..