An energy storage system based on battery and supercapacitors is presented. It allows bigger amount of intantaneous power. The properties of the proposed system are oriented in high efficiency, in a special topology with parallel channels. The paper presents also an active sharing device, for equalizing the voltages across a series connection of supercapacitor. Based on a buck-boost topology, this device ensures an optimal value for the stored energy, with a high equalizingciency.