A vehicle-mounted device for detecting battery state comprises a controller, a load unit, and a switch unit. The load unit is coupled to the battery, the switch unit is coupled between the battery and the load unit. The controller can control the switch unit to disconnect or connect the battery and the load unit at a predefined frequency. The controller can switch the load unit to generate ripple voltage on the battery, and measure the ripple voltage of each battery cell, an abnormal state of a battery can be determined according to the ripple voltage of each battery cell.