For body sensor networks (BSN) nodes, extending battery lifetime is one of the key problems. In this paper, we address the problems of designing battery-friendly packet transmission policies in order to maximize the lifetime of batteries for BSN nodes under certain delay and deadline constraints. We present local optimization scheme for slack time and evaluate it with respect to battery performance. The algorithm first simplifies the analytical battery model under the premise of ensuring battery model precision, and then distributes the available slack time between two adjacent tasks. The scheme was simulated for BSN nodes, and the results demonstrate a 79% reduction in the total charge consumption of six tasks in a BSN node along with the deadline constraint of 40 mins.