In Wireless Sensor Networks (WSNs), energy conservation is one of the main concerns challenging the cutting-edge standards and protocols. Most existing studies focus on the design of WSN energy efficient algorithms and standards. The standard IEEE 802.15.4 has emerged for WSNs in which the legacy operations are based on the principle that the power-operated battery is ideal and linear. However, the diffusion principle in batteries shows the nonlinear process when it releases a charge. Hence, we can prolong the network lifetime by designing optimized algorithms that reflect the battery characteristics. Within this context, this paper proposes a cross-layer algorithm to improve the performance of beacon enabled IEEE 802.15.4 network by allowing a Personal Area Network Coordinator (PANc) to tune its MAC behavior adaptively according to both the current remaining battery capacity and the network status. The performance of the new algorithm has been examined and compared against that of the legacy IEEE 802.15.4 MAC algorithm through extensive simulation experiments. The results show that the new technique reduces significantly the energy consumption and the average end-to-end delay.