Patent ID: 7884567

Claim:
A fuel cell system comprising: a fuel cell stack for producing power using an electrochemical reaction between fuel and an oxidizer; a secondary cell for storing and discharging power; a stack voltage converting circuit for converting a voltage produced by the fuel cell stack and for transferring the converted voltage to a load node, where an output terminal of the secondary cell and a load are connected to the load node; a voltage converting controller for controlling a conversion ratio of the stack voltage converting circuit so that a voltage on an output line of the stack voltage converting circuit and a voltage on an output line of the secondary cell are the same at the load node; a charging circuit for charging the secondary cell with power produced from the fuel cell stack; and a driving controller for controlling at least two output modes of the fuel cell stack and operation of the charging circuit, wherein the fuel cell stack is operated in one of the at least two output modes at which the fuel cell stack outputs power at different levels.