as for saving yourself a bunch of money I would recommend that you keep a battery tender on the vehicle. Me personally, i had one builtin under the hood so that all I have to do is open the hood a little and plug in an extension cord. then it will trickle charge. The vehicle uses a small amount of energy all the time to run things like the clock and other monitoring functions such as BMW assist if you have it. A battery tender will fix this issue.
I am not sure about the battery part as i have not yet researched that.