Isolation monitoring

An isolation monitoring approach according to the invention is defined with respect to first and second battery connection positions, each of which is not tied to a chassis. First and second respective resistor strings, each having a first end and a second end, are each connected at their first end to the respective first and second battery connection positions. The first and second resistor strings are each connected at their second end to the chassis. In each of the first and second resistor strings, an optically-controlled MOSFET relay is connected across part of the resistor string. Connected to each of the resistor strings, at a point between the MOSFET relay and the chassis, is a respective voltage measurement circuit connected to measure voltage relative to the chassis. In operation, the MOSFETs associated with the resistor strings can be turned on and off. This brings about an excitation signal with respect to the resistor strings, and the voltage measurement circuits can measure the voltages during the time of the excitation signal, comparing them with the voltages in the absence of the excitation signal. This permits arriving at an inferred value for isolation loss along a modeled leakage path between the associated battery connection positions and the chassis. In the event of an isolation failure, the event can be annunciated.