The self-discharge of lithium-ion cells is an undesired effect, because it directly influences the performance of batteries in electric vehicles. The aim of this manuscript is to illustrate the influencing factors for the self-discharge, especially the influence of time in which the cells stay unused. The self-discharge behavior of nickel manganese cobalt oxide (NMC)/graphite cells can be reproduced with three models for different time horizons: short, long, and overall time. The models are based on an approach originating from alkaline battery modeling. Some arguments justifying the validity of the approach are stated. The models are parameterized utilizing long time measurement data (160 days) obtained at different temperature and state of charge conditions. Limits of the models are discussed and quantified with simultaneous 99% confidence and prediction bands, that are constructed by means of nonlinear regression. A good agreement of the models and the measurements as well as narrow confidence and prediction band widths can be found.