A battery comprises an acid electrolyte in which a compound provides acidity to the electrolyte and further increases solubility of at least one metal in the redox pair. Especially preferred compounds include alkyl sulfonic acids and alkyl phosphonic acids, and particularly preferred redox coupled include Co³⁺/Zn⁰, Mn³⁺/Zn⁰, Ce⁴⁺/V²⁺, Ce⁴⁺/Ti³⁺, Ce⁴⁺/Zn⁰, and Pb⁴⁺/Pb⁰.