Battery lifetime is a critical design parameter in the operation of mobile computing devices. Maximizing the battery lifetime is a particularly difficult problem due to the nonlinearity of the battery discharge behavior and its dependence on the discharge profile. In this paper, we address the problem how battery aware dynamic voltage scaling policy optimizes the battery capacity consumption. We propose the procedure of slack distribution optimization to reduce the battery capacity consumption by distributing the slack optimally for the schedules generated by the existing battery aware DVS policies, and analysis the properties of the procedure. The experiment results show that the proposed procedure can further safe battery capacity consumption evidently.