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.

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).

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