I've come across two BMW owners over the past year who keep

They were probably diesels. If you don't drive diesels incredibly frequently, or in the winter, the gas needs to be heated so it doesn't solidify.

t. guy whose father had a 300D boat

Every modern car has many modules on a CAN bus that draw small amounts of electricity. Security modules that look for your key, infotainment that stays in a semi sleep mode for the system to load quickly when you open the door, memory for seat positions, etc. If you let a new S Class, A7, etc. sit for a month without being on a tender it will likely drain the battery to the point it won't start. A week should be no issue, but it depends on the car.

>>17536656
just to clarify since I didn't say it - because the gas needs to be heated there's a plug in the front of the car that when used heats the gas and keeps it from turning to jelly. We did that with the 300D all the time in the winter, though my dad had no fucking idea about it for a few months and my grandma that he got it from may have had no idea since she bought the car in '83 and never let him know, lol

>>17536669
I don't think those warrant plugging the car in or else every single car would come with that sort of plug

>>17536641
Pretty normal for anything with a thirsty security system or a small battery. You don't need to if you drive frequently though.

>>17536641
Are you sure they don't have engine block heaters?

>>17537568
Not sure - they both had upper end 3 series sedans, less than 4 years old. In fact, one of them was literally imported from Germany to America, so it has the seats reversed.

>>17537271
Most cars are expected to be driven regularly, but really any car battery should be hooked up to a tender if you're just letting it sit.

>>17537868
>Seats reversed.
What dou you mean by that?.

>>17536641
3 weeks and my nearly new GM car with its new high-capacity AGM battery is down to 40%. These things along with OnStar that broadcasts status al the time eats the batteries. You can't really take batteries down past 20% without sulphating risk. That means a practical real-life battery's actual bottom is 20% and not 0%. We all know that sulphated batteries suck ass.