We demonstrated that the performance of the polyoxometalate molecular cluster battery (POM-MCB) is significantly dependent on the weight ratio of POM in the cathodes; the discharge capacities per unit weight of POM decrease as the POM weight ratio increases. Operando XAFS and impedance measurements of the POM-MCBs with higher POM ratios revealed that a decrease in the number of POM reduction electrons due to higher resistance in the cathode causes smaller discharge capacities per unit weight of POM.