Recently portable mobile devices such as smart-phones and notebooks have rapidly increasing demands. Those devices consume more power because they are expected to offer more complex functionality including multimedia features. For these reasons engineering efforts are changing to focus on maximizing energy efficiency within a limited battery capacity instead of increasing computational performance. In this paper, we propose a battery management system using event driven programming technique on a embedded processor. We also show that the proposed system satisfies SBS (Smart Battery Specification) v1.1. The proposed system maintains minimum code size and memory size comparing to those of RTOSs. The proposed system can be also easily incorporated in the conventional RTOSs as a form of firmware.