This study investigated the effects of Na2SO4 additive in the positive electrode on the performance of sealed lead-acid cells. The additive Na2SO4 in the cured plates can reduce the 4BS crystal size, which produces a smaller α-PbO2 and β-PbO2 crystal size in the formed plates, which will have a larger surface area. The plate's chemical composition is independent of the amount of Na2SO4 additive in the positive electrodes. Plate composition relies only on the cure temperature conditions. Increasing amounts of Na2SO4 additive to the positive electrode will not decrease the crystal size appreciably. The optimal amount of Na2SO4 additive is 0.01–0.05 M, which produces the smallest crystal size and largest specific surface area. Cells with Na2SO4 additive in the positive plates have a smaller surface area, causing a higher initial capacity and average capacity per cycle for both testing methods: the standard cycle testing and the electric scooter (ES) driving pattern cycle testing. The initial capacity and average capacity can be increased up to 4% in the standard cycle testing and up to 8% in the ES driving pattern cycle testing.