Unpredictable battery lifetime arising from multitasking can have a significant negative effect on availability for mobile devices. In practice, some applications are prioritized and required to remain in operation for certain duration. This paper suggests a battery lifetime guaranteeing scheme for prioritized applications in multitasking mobile systems. The proposed approach profiles and analyzes the battery usage pattern of each task dynamically, and preserves the energy budget for operation of prioritized tasks for a guaranteed time. In addition, this paper proposes an energyconstrained scheduler that limits the energy consumption of tasks while preserving scheduling patterns, which translates to the QoS. The suggested scheme is implemen...