The capacity loss and coulombic efficiency of LiCoO2, LiFePO4, and LiMn2O4/graphite commercial Li-ion batteries have been examined using high precision coulometry. The cells were charged and discharged at different C rates and temperatures to observe trends in the aging of cells. The experiments show that time, not cycle count, is the dominant contributor to the coulombic inefficiency of Li-ion batteries cycled at low rates. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3487637] All rights reserved.