A cell with tall plates was simulated by two cells and the controlled voltage with time was successfully used to measure the response current–time curves. It indicates that the current in the upper part of the plate is much higher than that in the lower part in early stage of the discharge, but in late stage, it is reversed, especially at high discharge rates. The upper part of the plate has higher capacity and deeper depth of discharge (DOD) so that the active mass degrades and sheds more quickly in cyclic applications. In the initial discharge, the lead sulfate formed at positive plates produces the internal stress and enhances the electric connection among the active mass particles. But the positive active mass only contributes a little to the conductivity of plates. The negative active mass with a bit shrinking but no passivation has better conductivity than the expanded active mass. The higher ohmic polarization of the active mass appears at higher discharge rates, in the upper part of the plates, in the late stage of the discharge, in aged battery, especially for negative plates.