This paper presents a buck-boost converter with high-efficiency and smooth-transition to extend the battery life of portable devices. The new control topology minimizes the switching and conduction losses at the same time even four switches are used. Therefore, over a wide input voltage range, the proposed buck-boost converter with minimum switching loss like the buck or boost converter can reduce the conduction loss through the use of the reduced average inductor current (RAIC) level. Especially, the converter offers good line/load regulation and thus provides a smooth and stable output voltage without being affected by the decreasing battery voltage. Experimental results show that the output voltage is regulated for the whole battery life time and the output transition is very smooth during mode transition. Besides, the output voltage ripple and the average inductor current are effectively minimized by the proposed control scheme. The peak efficiency is 97% and output voltage ripple is reduced from 100mV to within 10mV.