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2001 BMW 740 - Dead battery from sitting?

I took this car to FL to have a car when I go down there. It sits for six weeks or so. The battery is dead when I get there and is so dead it won’t even jump. I took it to have it charged and it held once, but the second time, the battery went dead again and wouldn’t hold a charge. The battery is less than a year old. Did letting it sit cause the battery to go bad? Should I take off both terminals? If the battery is dead, it’s a hell of a time getting in the trunk as the spring loaded trunk closer is electric!

See if there is parasitic drain. To me it seems it should hold sufficient charge to start.
Alternatively, get a battery tender to maintain the battery.
It is possible for a new battery to go bad. Totally draining a battery will hasten its death.

These cars are known for this even in 2001. There are a lot of computers in this car and nearly all of them don’t completely shut off when the car is shut off. So all of them are drawing just a little bit of battery when the car just sits. It is very possible on a car this old for one or more of these computers to have developed an internal short that is draining the battery or keeping that computer away drawing more power than it should.

As @Purebred says - parasitic drain. Repair shops can test for this. Expect to pay for their time trying to find the cause. Expect to pay a large sum of money to replace the offending part if you can even get one for this 19 year old car.

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If you want a cheapo fix get a battery maintainer to use while it sits.