Since 1985, we have been developing the redox flow battery, which is a kind of new-model secondary battery for stationary energy storage. In 1986 we manufactured the 10kW class system and evaluated it by using the iron/chromium electrolyte. Furthermore, we installed a 60kW class system on Kansai Electric Power Company's Tatsumi substation. This system was connected to a 6.6kV power distribution system and tested for five years. It achieved 1,819 charge/discharge cycles, exceeding the target of 1,500 cycles. On the other hand, we also developed the cell which used the vanadium electrolyte whose electromotive force was higher than the iron/chromium electrolyte. This paper reports the principles and configuration of the redox flow battery, the test results of the 450kW battery system and the improved 20kW-8h battery system. The steps toward the realization of the redox flow battery are shown.