A study is made of the effect of unreacted monomer on the performance of lithium-ion polymer batteries that use cross-linked polyethylene glycol diacrylate-based polymer electrolytes prepared by free radical polymerization. At high discharge rates and low temperature, the discharge capacities of test cells containing the polymer electrolytes prepared at various concentrations of the initiator decrease as the content of the unreacted monomer in the polymer electrolytes increases. This behaviour is consistent with an increase in the interfacial resistance of the test cells with increase in the content of the unreacted monomer The source of the high interfacial resistance is considered to be due to reaction of the unreacted monomer at the electrode surface to form a resistive film during charging for cell formation.