The 36 or 48V valve-regulated lead-acid (VRLA) battery packs have been widely applied to the power sources of electric bicycles or light electric scooters in China. The failure modes of the 12V/10Ah VRLA batteries have been studied by the cycle life test at C2 discharge rate and 100% depth of discharge (DOD). It indicates that the main cause of the battery failure in this cycle duty is the softening and shedding of positive active mass (PAM) rather than individual water loss, recombination efficiency or sulfation, etc. When the electrolyte saturation falls to a certain extent, the high oxygen recombination current leads to the depolarization of the negative plate and the shift of the positive plate to a higher potential. The violent oxygen evolution accelerates the softening of PAM and the end of cycle life.