A battery device of the invention has a high-voltage battery unit, which includes three battery modules C1 through C3, three voltage equalization circuits B1 through B3, and six switches SW1 through SW6. Each of the three battery modules C1 through C3 includes plural lithium secondary cells arranged in series. Each of the voltage equalization circuits B1 through B3 works to equalize the voltages of the respective cells included in a corresponding one of the battery modules C1 through C3. The six switches SW1 through SW6 are individually switched on and off to switch over the connection state of the battery modules C1 through C3 between serial connection and parallel connection. In the battery device of the invention, the voltage equalization process activates the voltage equalization circuits B1 through B3 to respectively equalize the voltages of the plural cells included in each of the three battery modules C1 through C3 in the state of serial connection of the battery modules C1 through C3. After completion of the voltage equalization in each of the three battery modules C1 through C3, the voltage equalization process connects the three battery modules C1 through C3 in parallel and equalizes the voltages of the respective battery modules C1 through C3. This arrangement effectively reduces the power loss by the voltage equalization and thus enhances the total energy efficiency of the whole battery device.