A simple ageing model is proposed for the fading of lithium–sulfur (Li–S) cell capacity with the number of cycles. The model could be applied for the ageing description of cells with an unknown internal structure in a wide range of engineering applications. Cycle profile that patterns with the real-life operation of cells is proposed. Thus, the degradation of the cell is simultaneously influenced by the 100% depth-of-discharge cycling and the cell staying at a charged state. The proposed model is described by the differential equation considering that the decrease of cell capacity per cycle is proportional to the product of cycle number and the charge available to be discharged within the appropriate cycle. The model covers the degradation processes dominant in the initial stage of ageing, where the cell capacity decreases from 100% to about 25% of its initial value. The solution of the proposed differential equation is an analytical function that requires only three fitted parameters. The dependence of model parameters on the charge/discharge current rate is shown for studied Li–S cells. Possible sources of degradation, such as the decrease of electrode effective area and the decrease of potential barrier on the electrode/electrolyte interface, are determined from the evaluation of the coulombic efficiency of the charge/discharge process.