Prismatic type judged as most suitable for maintaining properties for 20 years
The storage battery system is placed in 10 containers (Fig. 5). A total of 3,200 storage battery modules (Fig. 7) consisting of 12 cells are housed in the containers (Fig. 6). Each cell has a voltage of 3.7V and a capacity of 47.5Ah. Two storage battery systems, consisting of 16 modules of serial connections arranged in 10 rows, are housed in each container. Each of the storage battery systems is connected to one 500kW power conditioner, and 20 power conditioners are housed in three containers.
As for the service life, the system was designed to maintain its properties for 20 years. Additions will be carried out by checking the battery capacity after 15 years by a service life simulation.
The history of Li-ion storage batteries is short and "there is no case of service life, measured after 15 years or longer of actual use of Li-ion storage batteries," although the company has offered Li-ion storage battery systems for a relatively long time, Hashimoto said. The battery characteristics need to be improved to increase the service life. For this purpose, "it was determined that the prismatic cell is the most appropriate" in respect to the battery shape, GS Yuasa said.
The cost of prismatic cells is higher compared with laminate type cell and cylindrical cell. However, leakage, which is sometimes observed in laminate type cell, is prevented by enclosing the components, including power and electrolyte, in a metallic square casing and sealing the casing by welding. Furthermore, accumulation of waste heat at the center, which sometimes occurs in cylindrical cell, does not occur in prismatic cell. And "it has been confirmed that they feature high performance and safety," to Hashimoto said. The performance and safety of prismatic cell were evaluated by the project this time.
GS Yuasa also developed BMS (battery management system) in the project. Special attention was paid to "accurate measurement of SOC (state of charge), cell voltage, module voltage, cell temperature, etc." In this regard, the industrial storage battery system of the company incorporates large cells and actually measures all cells, not in group units, and transmits the data to the upstream system to ensure perfect control.
The system has been in operation since April, and the cases of exceeding the fluctuation range of "1% per minute" have been "controlled to only a few cases," Hashimoto said.