This work provides an aging study of a graphite vertical bar LiNi1/3Mn1/3Co1/3O2 (NMC) Li-ion pouch cell with a nominal capacity of 10 Ah. By means of resistance and capacity measurements the cell's behavior is tracked over time under consideration of temperature and cell voltage impact. Tests duration was up to 15 months effective testing time. Observed effects and possible aging mechanisms are discussed considering the results from capacity and resistance measurements. The test results are used for a calendar lifetime prediction. In addition to a detailed calendar life study, also cycle life tests are discussed briefly to point out additional aging effects based on cycling. The paper may also serve as data source on aging for further work on battery lifetime modeling and battery diagnostics. Selected cells were taken from the aging tests and were used for a detailed post-mortem analysis (see paper "Part B: Post-Mortem Analysis"). (C) 2013 Elsevier B.V. All rights reserved.