Ditto...a trickle or float charger does the trick even when sitting for long periods of time with the battery connected.
BMW has the consumer cut-off which should leave very little electrical draw...enough for a trickle or float charger to keep things up to snuff.
But disconnecting the battery isn't a bad thing...it just makes extra steps to hop in the car to take her for a spin when you do get ready to drive her.
