First-principles methods have started to be widely used in materials science for the prediction of propel-ties of metals, alloys, and compounds. In this study, we demonstrate how first-principles methods can be used to predict the average open-circuit voltage that can be obtained from a lithium battery with spinel or layered manganese oxides used as the cathode. For this purpose we combine a basic thermodynamical model with the ab initio pseudopotential method. The good agreement between the computed and experimental average output potentials suggests that first-principles methods can be an important tool to design novel battery materials.