MHI and Kyushu Electric Power have been carrying out a collaborative charge and discharge*1 test on one of the world’s largest (24 kWh) lithium-ion battery energy-storage systems. The test has surpassed the one year mark without trouble and has achieved 380 continuous charge and discharge cycles. Under license from the Ministry of Economy, Trade and Industry as the nation's first power plant to use lithium-ion batteries, the system at MHI's Nagasaki Research & Development Center has been connected to electricity grid and operated since September 2003. Such a prolonged and continuous grid-connection charge and discharge is unprecedented. The operating conditions were so good, that, from here on in, we will substantiate and build on the charge and discharge test, with an aim of entering the market within the next few years. The lithium-ion battery energy-storage system has been formed by serially connecting 88 individual 270 Wh-class lithium-ion batteries. By recharging at night with the less expensive surplus electricity and then discharging during the day, the system contributes to more even loadings, and can cut down on electricity costs. The electricity capacity of the system is 24 kWh. The energy-storage system boasts the world's best electric power conversion efficiency ratio at 86%. The lithium-ion battery has been developed using the exceptionally safe mangan-oxide, and it has a world-class energy density of 160 Wh/kg. Endurance testing of the battery elements is underway and has exceeded 2,300 cycles. It is expected that the 3,500 target cycles will be achieved. The system has been developed with small-scale power users in mind: small stores, offices and homes. Separate energy-storage systems using Redox flow batteries*2 and NaS*3 batteries are being made suitable for practical use by larger-scale users, but there had been a greater demand from small-scale power users for a new high-performance easy-to-use energy-storage system. For this reason, in 2000, development was launched into an energy-storage system that made use of the high-capacity long-life lithium-ion secondary batteries. Then in March last year, the highly efficient 24 kWh-class prototype energy-storage system, with minimum charge and discharge loss, was successfully manufactured. From last autumn, operational results of the system have been built up through the grid-connected tests. A wide range of applications are envisaged in the future for this high-capacity, long-life small-scale system, which emits neither exhaust gas nor noise, is maintenance free, and which has the outstanding function of being a source of emergency power. The shipping of sample products is in its sights, and assessments for turning the system into a full-scale commercial operation are moving forward.
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