Storage cells such as secondary batteries and capacitors are used in power storage systems such as instantaneous voltage drop/instantaneous power failure compensation apparatuses and power storage apparatuses for railroad use. In such power storage systems, since an available voltage of a single storage cell is only several volts, a plurality of storage cells are used in a series-connected manner.
In such a case, it is more efficient to prepare a plurality of storage modules each including a certain number of storage cells connected in series and obtain a target usage voltage by connecting a required number of storage modules in series. With the storage cells being modularized into modules each including a certain number of series-connected storage cells, it becomes possible to standardize the modules and to reduce cost by mass production and thus users can easily and freely create power storage systems of desired voltages using the standardized modules.
In case voltage variation occurs between cells in series-connected cells in which a plurality of storage cells are connected in series, there arises a problem that the voltage concentrates on a specific cell and the duration of the cell will be shortened. Such a problem due to voltage variation between the cells becomes more significant as the number of cells in the series connection increases.
Therefore, use of storage cells in a series connection requires a balance correcting circuit that compensates for voltage variation between the cells. Various techniques have been suggested for balance correcting circuits that compensate for voltage variation between the cells (E.g., see Patent Literatures 1 and 2).