I think the recorder is marked 12 to 30v but it's positive to case!
If you just want to run the Nagra from a car battery on its own, then you just need a connector with a cable (and an in-line fuse) and a floating battery can be grounded at either end
But if the battery is still conected to the car or anything else which forces negative ground, then you'll need an inverter to reverse the polarity
Or you need a classic car
David