An apparatus and method for measuring a voltage across two ends of each of a plurality of secondary batteries included in a battery module. The battery module further includes a busbar. The busbar is electrically connected between a negative terminal of one of the plurality of secondary batteries and a negative terminal of the battery module. The apparatus includes a plurality of voltage input terminals electrically connected to the two ends of each of the plurality of secondary batteries to receive the voltage across the two ends of each of the plurality of secondary batteries. One of the plurality of voltage input terminals is a reference voltage input terminal electrically separated from a first reference ground. The reference voltage input terminal is electrically connected to a second reference ground connected to one end of the busbar to receive voltage applied to the second reference ground.