Lithium-ion cell-to-cell variation during battery electric vehicle operation

484 new and 1908 aged lithium-ion cells out of two identical battery electric vehicles (i.e. 954 cells each) were characterized by capacity and impedance measurements to yield a broad set of data for distribution fit analysis. Results prove alteration from normal to Weibull distribution for the parameters of lithium-ion cells with the progress of aging. Cells with abnormal characteristics in the aged state mostly exhibit lower capacities as compared to the distribution mode which is typical for the left-skewed Weibull shape. In addition, the strength of variation and the amount of outliers both are generally increased with the aging progress. Obtained results are compared to vehicles' operational data to provide recommendations with the aim to minimize the increasing parameter spread. However, neither temperature gradients in the battery pack nor an insufficient balancing procedure were determined. As the appearance of cells with suspicious parameters could not be assigned to local weak spots of the battery pack, a random and inevitable type of origin is assumed. Hence, the battery management system must ensure to detect outliers in a reliable manner and to balance resulting drifts of cells' states of charge to guarantee a safe battery storage operation. (C) 2015 Elsevier B.V. All rights reserved.