Solid polymer electrolytes (SPE) based on poly (vinyl alcohol) (PVA) and sodium bromide (NaBr) complexed with sulfuric acid (SA) at different weight percent ratios were prepared using solution cast technique. The structural properties of these electrolyte films were examined by XRD studies. The XRD data revealed that sulfuric acid disrupt the semi-crystalline nature of (PVA)0.7(NaBr)0.3 and convert it into an amorphous phase. AC conductivity and dielectric spectra of the electrolyte were studied with changing sulfuric acid content from 0.87 to 3.4molesl−1 (M). The highest conductivity of (PVA)0.7(NaBr)0.3 matrix at room temperature was 1.12×10−6 Scm−1 and this increased to 6×10−4 Scm−1 with doping by 2.6M sulfuric acid. The electrical conductivity (σ) and dielectric permittivity (ɛ′) of the solid polymer electrolyte in frequency range (100Hz to 100kHz) and temperature range (300–400K) were carried out. The electrolyte with the highest electrical conductivity was used in the fabrication of a magnesium battery with the configuration Mg/SPE/MnO2. The fabricated cells give capacities of 4.85mAhg−1 and have an internal resistance of 900Ω.