Solid polymer electrolyte (SPE) is synthesized by solution casting technique. SPE uses poly (vinyl alcohol) PVA as a host matrix doped with solid acid NaHSO₄, ethylene carbonate (EC) as plasticizer and (Si) as filler. Polymer electrolytes are characterized by Impedance Spectroscopy (IS) to determine the composition of the additive which gives the highest conductivity for each system at room temperature. The ionic conductivity of the polymer electrolytes increases with temperature and obeys the Arrhenius law. X-ray diffraction (XRD) studies indicate that the conductivity increase is due to an increase in amorphous content which enhances the segmental flexibility of polymeric chains and the disordered structure of the electrolyte. Scanning electron microscopy (SEM) images show the changes morphology of solid polymer electrolyte. A magnesium battery has been fabricated with optimized composition (3.75wt. % Si) polymer electrolyte using FeS₂ as electrode which gives real capacity 112 mAh/g and has an internal resistance of 160Ω.