Proton-conducting gel electrolytes with acid immobilized within a covalently cross-linked polymer network and composites containing the gel electrolytes provide low ionic resistance, minimize acid stratification, and prevent dendrite growth. The gel electrolytes can be formed from monomers dissolved in concentrated sulfuric acid and subsequently covalently cross-linked between the battery electrodes, or the covalently cross-linked gel electrolytes can be formed in water and subsequently exchanged into sulfuric acid. The mechanical properties of these gels can often be enhanced with the addition of silica powder, silica fiber, or other additives. In some cases, the covalently cross-linked gel electrolytes are formed in the presence of a conventional silica-filled polyethylene separator or within a low density fiber mat to provide mechanical reinforcement and controlled spacing between the battery electrodes. The covalently cross-linked gel electrolytes provide low ionic resistance, and increased power capacity of the battery, because the polymer networks can be formed at low concentrations (