A theoretical description of the thermodynamic properties of the battery systems : Al/NaCl-AlCl3-Al(2)X(3)/Ni-felt (X = S, Se, Te) and the corresponding system without chalcogen has been provided for cells with basic to slightly acidic NaCl-AlCl3 melts containing small amounts of chalcogen. The model developed describes the equilibrium concentrations of constituent species in the electrolyte and equilibrium potentials of the electrodes vs. number of coulombs passed through the cells. For cells without chalcogen curves were calculated under the assumption of NiCl2 formation showing corresponding Variation of anode potential, cathode potential, and cell voltage as a function of electrolyte composition. For sulfide containing cells the plateau of lowest potential has been found to be associated with essentially pure nickel sulfide, NiySz. A procedure for model fitting to the cathode potential curve-form of the NiySz-plateau is presented. A general equation is proposed for the reaction taking place along the second plateau.