This battery monitoring system monitors a battery unit including a plurality of battery packs that include a first battery pack. The first battery back includes a plurality of cells. The battery monitoring system comprises: a first battery management device that manages the first battery pack; and a current sensor that detects the charge/discharge currents of the plurality of cells and outputs, to the first battery management device, a detection signal from the detection. The first battery management device comprises: a voltage acquisition unit that acquires the voltage between prescribed positions on a current path of the first battery pack; a current acquisition unit that acquires the charge/discharge currents of the plurality of cells through the current sensor; and a state estimation unit that estimates the state of the first battery pack using the voltage acquired by the voltage acquisition unit and the charge/discharge currents acquired by the current acquisition unit.
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