In this paper, an aging estimation method is proposed for the lead-acid batteries serially connected in a string. This method can prevent the potential battery failure and guarantee the battery availability, and it can serve as an indicator for aging or degradation of the lead-acid battery. The salient feature of the proposed method is that aging of the individual battery is estimated automatically at the end of each discharge cycle by only measuring individual battery voltage in a string of batteries, so that no complicated measurement is required. To verify the proposed aging estimation method, aging experiments for lead-acid batteries are developed. The experimental results show that the proposed aging estimation method provides the expected results.