In a battery-powered surveillance system, event characteristics, e.g., arrival time, duration, etc, vary in unpredictable manner and the battery lifetime nonlinearly depends on both discharge current intensity (rate-capacity effect) and idle period (recovery effect). In this work, we present a novel video encoding method in a battery-powered surveillance system which adjusts video operation mode so as to minimize the recorded image distortion exploiting uncertain event characteristics and nonlinear battery characteristics. The propose method overcomes both pessimistic assumption on event characteristics and unrealistic assumption on battery characteristics while meeting the system requirement on battery lifetime. Compared to the conventional method [18], experimental results show that the proposed method provides up to 3.02dB improvement on image quality in H.264 encoder.