Monitoring device and method for monitoring a closed state of a battery switch, inverter device, electric axle drive and motor vehicle

A monitoring device for monitoring an on/off state of a battery switch that has a power source, a diode connected in series to the power source, and an evaluation circuit. The power source and diode are connected in parallel to the battery circuit. The power source is configured to generate a current or voltage between a first terminal and second terminal of the battery switch. The evaluation circuit is designed to create an evaluation signal based on the voltage between the diode and the power source that indicates the on/off state of the battery switch.