UC Davis

As lithium-ion battery (LIB) cells degrade over time and usage, it is crucial to understand the remainingcapacity, also known as State of Health (SoH), and inconsistencies between cells in a pack, also known as cell-to-cell variation (CtCV), to appropriately operate and maintain LIB packs. This study outlines efforts to model pack SoH and SoH CtCV of nickel-cobalt-aluminum (NCA) and lithium-iron-phosphate (LFP) battery packs consisting of four cells in series using pack-level voltage data. Using a small training data set and an under 3-minute testing procedure, partial least squares regression (PLS) models were built and achieved a mean absolute error of 0.38% and 1.43% pack SoH for NCA and LFP packs, respectively. PLS models were also built that correctly categorized packs having low, medium, and high ranked SoH CtCV 72.5% and 65% of the time for NCA and LFP packs, respectively. This study further investigates the relationships between pack SoH, SoH CtCV, and the voltage response of NCA and LFP packs. The slope of the discharge voltage response of the NCA packs was shown to have a strong correlation with pack dynamics and pack SoH, and the lowest SoH cell within NCA packs was shown to dominate the dynamic response of the entire pack.