The distributions of current density and potential of automotive negative plate were studied by measuring the IR drop in the H2SO4 solution between the positive and negative plates. At the beginning of discharge, the distributions of current density, potential and polarization resistance are uniform. In the later stage, high polarization appears at the top and bottom of the negative plate and the current density falls very quickly in these regions. Therefore, high polarization resistance of the active mass increases very quickly at both the top and bottom of the negative plate. It is mainly caused by the passivation of the negative plate which obviously decreases by constant current charge. At low temperature, at the end of 3 C discharge, the highest polarization and the lowest current density appear farthest away from the lug and at the top of the negative plate.