Polyethylene oxide (PEO) based-solid polymer electrolytes were prepared with low weight polymers bearing carboxylic acid groups added onto the polymer backbone, and the variation of the conductivity and performance of the resulting Li ion battery system was examined. The composite solid polymer electrolytes (CSPEs) were composed of PEO, LiClO4, PAA (polyacrylic acid), PMAA (polymethacrylic acid), and Al2O3. The addition of additives to the PEO matrix enhanced the ionic conductivities of the electrolyte. The composite electrolyte composed of PEO:LiClO4:PAA/PMAA/Li0.3 exhibited a low polarization resistance of 881.5 ohms in its impedance spectra, while the PEO:LiClO4 film showed a high value of 4,592 ohms. The highest ionic conductivity of 9.87 × 10−4 S cm−1 was attained for the electrolyte composed of PEO:LiClO4:PAA/PMAA/Li0.3 at 20 °C. The cyclic voltammogram of Li+ recorded for the cell consisting of the PEO:LiClO4:PAA/PMAA/Li0.3:Al2O3 composite electrolyte exhibited the same diffusion process as that obtained with an ultra-microelectrode. Based on this electrolyte, the applicability of the solid polymer electrolytes to lithium batteries was examined for an Li/SPE/LiNi0.5Co0.5O2 cell.