Ditto...back in the 60s prior to the introduction of electronic modules controlling many of the car's functions...disconnecting the battery with the engine running was plausible.

But now...with all the CPU controlled electronics in modern vehicles...its not a very good idea. See an exert from one of my BMW's owners manual:

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• 2015 f06 650ix GC (Moonstone exterior/Cohiba brown Merino Leather interior)
• 2014 f15 X5 50i (Space Gray exterior/Mocha Nappa Leather interior)