A 16kWh power storage system applying Li-ion batteries has been jointly developed by Kyushu Electric and Mitsubishi Heavy Industries. The system comprises a Li-ion battery pack, an inverter and a battery protection system. Four cells, each with a capacity of 270Wh, are connected in a series to form a single 1.1kWh module battery. The Li-ion battery pack is made up of a series of 15 such module batteries. A 270Wh cell, in which Li–manganese oxide is used for the cathode and graphite for the anode, can expect to perform 3500 lifecycles during operating conditions of the rated 70% DOD. Even though power loss in the inverter and battery protection system must be further reduced, satisfactory response was found both in the batteries and inverter control, leading to the creation of a practical power storage system applying Li-ion batteries.