The effects of cathode/anode capacity ratio on the performance of mercury-free alkaline zinc-manganese dioxide batteries were studied. An alternating current impedance analysis and a continuous discharge test with constant electrical load were employed. The discharge and leakage performances of alkaline batteries varied strongly with capacity ratios and discharge rates. The lower capacity ratios favored discharge performance at high rates, whereas discharge performance at low rates depended only on the quantity of manganese dioxide present. In addition, the lower capacity ratios yielded high in-cell gassing with severe leakage problems, especially at partially discharged cell.