[farrell]

will changing my auxilliary battery help my car hold its charge for more than 2 days at a time?if not what exactly does it do?car has a new main battery and alternator btw

Mmm-Five
Not all M5 have the aux battery option.
Those vehicles with lots of electrical options or cold climate country tend to have the offering of aux battery.
Its stored in the trunk behind the lh trim panel from memory.
Jonsmiff
If your vehicle is discharging the battery at the rate you describe, then something electrical has gone open circuit & is drawing vast nos of milliamps per hour.
Get a vehicle sparky to look at the alarm & wiring first. Then progress through the other systems looking for the problem.
Good sparkys will have some idea of where to go first & will check earths,
Alternator , starter ,interior / boot lights etc.

One thing is for sure, your aux battery is not going to make any difference.

Good Luck
Farrell