This circuit continuously checks whether traction grid and 12V grid are isolated against each other.

The 22k resistor on the top left connects to a tap at the “electrical” center of the traction battery's cell chain. If the isolation is flawless, this point will be kept at 0V, referred to the 12V grid's ground. The traction battery is then “balanced”, with the positive rail at around +50V, and the negative rail at -50V referred to 12V grid ground.

In case of an isolation fault, the center tap will be forced to a potential different from 0V. This results in a current across the 22k resistor. This current is smoothed by a capacitor and rectified, and will eventually trigger the solid state relay on the top right of the diagram. Currently, the relay will be triggered at around 12V offset of the center tap.

The output of the relay enables a simple oscillator, that drives a beeper at around 4 beeps per second.

Below a view of the circuit board. The photos are intentionally shrunk because it grew so ugly.

The “isolation check” circuit is continuously powered (i.e. not dependent on ignition). It's function can always be verified by a test button at the rear side of the rear battery box, that simulates a slight isolation fault (see chapter “rear traction battery box” below).