OP this is exactly what I did on my old Car and still do in the MINI. I have an old 30pin iPhone and snap in adapter and leave it in the car all the time. I have never had an issues with the car battery going flat.
I would still be doing it in the M2 if it was not for the fact my car has wireless charging in place of the snap in adapter
Instead I purchased a 256GB USB stick and transferred my entire music collection to that. The 13GB in the car head unit was just too small!