Battery monitoring device and power state monitoring method

A power state monitoring method of a battery monitoring device for a battery including a plurality of battery stacks, in which the battery monitoring device includes a plurality of monitoring modules and a power supply device, each of the monitoring modules is disposed in a corresponding one of the battery stacks and is configured to monitor a state of the corresponding one of the battery stacks, the power supply device includes a plurality of first power lines, a first switch, a first capacitor, a second power line, a plurality of second switches, and a plurality of second capacitors, each of the first power lines includes a first positive electrode line and a first negative electrode line, each of the first power lines is connected to a corresponding one of the monitoring modules so as to supply a power to the monitoring modules, the second power line includes a second positive electrode line and a second negative electrode line, the second power line is configured to receive a predetermined amount of power, the first capacitor is configured to be connected to the second positive electrode line and the second negative electrode line of the second power line via the first switch and to be connected to the first power lines via the second switches, and each of the second capacitors is configured to be connected to the first positive electrode line and the first negative electrode line of corresponding one of the first power lines, the power state monitoring method comprising determining a state of power supply from the power supply device to the monitoring modules by using an increase amount in a voltage of the first capacitor of when the first capacitor is charged.