The dependency among cells in the battery pack is an objective phenomenon, which can affect the overall degradation, cycle life and reliability of battery packs. To solve this problem, the degradation tests of four configuration lithium-ion battery packs are carried out. The dependency degree of four battery packs is discussed by comparing the parameters difference degree among cells in the charge-discharge profile. The results show that the degradation processes of a battery pack and that of cells in the pack are dependent. The dependence degree of a battery pack is positively correlated with the degradation rate of a battery pack. After eliminating collinearity of independent variables by Principal Component Analysis, the degradation process-dependent model is established by the Linear Regression Model. High regression coefficient (R-2 > 0.9) and p-value < 0.0001 of models indicate that the degradation process dependency can be quantified. The results can provide a theoretical and methodological basis for designing, managing and optimizing battery packs in practice.