A method and system for energy management of a composite battery are provided to control and manage products of reaction going on in the composite battery. During the reaction, the gas products are usually exhausted and wasted. If the gases could be recycled, battery effectiveness would be improved. According to the present invention, the gases are collected and then recycled by an exhaust gas recycling device. In addition, the method and system involve analyzing data of the generated electrical energy, data of the produced gases, operational data of the composite battery and device data for the exhaust gas recycling device. Thus, the composite battery is controlled and managed according to the analysis, and its effects are improved.