An apparatus for detecting the state of a checkable storage battery in a vehicle having at least two vehicle onboard power supply systems of different operating voltages is provided. The vehicle onboard power supply systems are coupled by a DC/DC converter, which converts a first DC voltage of a first vehicle onboard power supply system into a second DC voltage of the storage battery in the second vehicle onboard power supply system. The vehicle onboard power supply systems have a control device that includes a test signal module configured to supply a test signal via the DC/DC converter to the checkable storage battery, a measured value detection module configured to measure the reaction values of the checkable storage battery, and an evaluation module configured to detect the state of the storage battery from the measured reaction values.