Patent attributes
A battery monitoring unit and sensors are supplied with a common operating supply voltage to operate. Detection values from sensors are sent to a control circuit by the battery monitoring unit. The control circuit monitors the operating supply voltage using a voltage sensor and additionally determines whether or not the detection values from the sensors before the relay connection falls within a normal range to more accurately detect a power supply reduction abnormality of the battery monitoring unit. Therefore, abnormalities can be detected accurately when the sensor and the battery monitoring unit provided for the secondary battery cannot operate normally due to a reduced operating supply voltage.