Basic studies to clarify the premature capacity loss (PCL) mechanism have been carried out. It was found that PCL is a phenomenon due to the increase of positive plate resistance, especially the interfacial resistance between the positive grid and the positive active material (PAM) by PbSO4 formation in the corrosion layer. PCL occurred when the adhesion between the grid and the PAM was poor, H2SO4 concentration at the interface between the grid and the PAM is high and the corrosion layer mainly consisted of beta-PbO2.