Premature capacity loss is a severe problem observed in lead/acid batteries; it has been localised at the grid/positive active material interface (PAM) and in the PAM. In order to understand these phenomena, cycled batteries with Pb-Sb-Sn and Pb-Ca-Sn positive grid alloys were studied and compared with floated batteries with Pb-Ca-Sn positive grid alloys. The evolution of the crystallographic and morphological structure of the PAM and of the grid/PAM interface during the tests were investigated. Formation of isolated agglomerates of PAM by the phenomena assimilated to sintering was found to be the reason of the failure of cycled batteries, whereas the intergranular corrosion of the grid initiated the floated batteries' end of life. The results and mechanisms of these phenomena are presented.