The discharge characteristics of a Li/Li y CoO2 cell were simulated by numerical calculations. Based on the program proposed by Newman's group, the change in entropy due to lithium-ion insertion into the active materials was introduced into the open-circuit potential (OCP) for more practical applications. The OCP of LiCoO2 (y =1) at a full-discharge state was 3.82V, which was determined from the original program. However, it should show a rapid decrease to minus infinity when discharge is complete (y =1), which is due to the introduced entropy term. In addition, the local use of active materials across the Li y CoO2 electrode was not uniform, whereas the original program showed a constant value throughout the electrode. Based on the present results, the introduction of an entropy term to the OCP of Li y CoO2 works reasonably well for more practical applications.