A battery-state monitoring system for precisely and efficiently estimating the state and service life of each of a plurality of storage batteries while suppressing variations in voltage between the batteries in a large-scale power-supply system that is provided with the storage batteries charged with power generated by utilizing natural energy is provided. In addition, the recess formed at the surface edge of the LCD module is combined with a fastening hook formed at the rear case so that the LCD module can be secured to the rear case and to the front case without any separate fastening member such as a stud or insert. The screw is pre-assembled and the support element is substantially irreversibly connected to the base body. The protection device includes a reset interface for transmitting a reset signal to the reset module to enable the communication device to release activity.