RE: "Now the question I have for the electronic wizards is "what is better for an X5? A battery connected and trickled charged constantly or a vehicle with the battery disconnected from the electrical system but on a tender?" I am beginning to think that having everything activated creates a potential issue long term on stored vehicles. Most car collections use a quick connect at the battery while on a charger standard practice. Thoughts? Or should I move this question to the SAE forum as it is totally theoretical?"
^ Not an electrical tech here...old shade tree mechanic, and purely anecdotal.
Fwiw, I have had the '01 X or the '02 VetteVert on a Battery Tender brand maintenance charger for many years.
One of those cars is in the garage,
in situ for several months per year.
I don't run the BT 24/7; I simply run it through an old fashion mechanical appliance timer set to come on for ~24 hrs a couple times per week, with the BT alligator clips attached to the underhood lugs, and electrical taped snugly so the passing mouse can't knock off a clip.
Have never had an 'electrical prob'/dead batt in either car over many years of doing this winter regime, to either of them. Whichever car it is, starts & runs fine 5 months after sitting.
I try not to overthink this stuff too much...
GL, mD