In an apparatus, an input switch circuit selectively connects one of unit batteries to a capacitor. A first group of at least one unit battery has a first polarity when being connected to the capacitor. A second group of at least one unit battery has a second polarity when being connected to the capacitor. A voltage converter converts a voltage of a unit battery selectively connected to the capacitor into a value within a voltage range manipulatable by a voltage monitor. A reference voltage setter variably sets a reference voltage according to one of the first polarity and the second polarity of the voltage of a unit battery selectively connected to the capacitor. The reference voltage is used as a reference to match a scale of the voltage of a unit cell selectively connected to the capacitor for each of the first and second polarities with the predetermined voltage range.