Abstract: An alarm device of a vehicle battery for monitoring voltage ranges in charging and discharging processes of the vehicle battery uses a sampling circuit to monitor the charging and discharging voltages real time of a battery, and prompts alert through a light or a buzzer of an alert unit in accordance with the preset voltage stage in a charging and discharging process of the battery. Moreover, a connection port compatible to an external electric device for accessing and examining the defect of a malfunction battery as a reference for maintaining and updating a malfunctioned battery.