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Old 08-20-2011, 07:50 AM
timfitz63's Avatar
timfitz63 timfitz63 is offline
MT = Doppler Shifting
Location: Lorena & San Antonio, TX
Join Date: Aug 2010
Posts: 930
Mein Auto: 2007 X3 (MT)
Two weeks of idle time in the garage is a bit quick for a relatively new battery to discharge. Could be a problem with the battery itself (it's not outside the realm of possibilities that the battery was defective), but it's probably more likely there's a drain being applied by the vehicle.

The computer (ECU) of most modern vehicles uses the 'door lock' command to put the vehicle into 'sleep' mode. So it's generally a good practice to lock the doors, even inside your own garage, and particularly if you know the vehicle will sit for a while.

One thing I kind-of figured out on my own with C5 Corvettes (and it may apply to BMW's as well) is that you need to remove the key from the ignition when the vehicle sits idle for lengthy periods, or it will draw down the battery much more quickly; I managed to almost completely draw down the battery of my C5 simply by leaving the key in the ignition for a week or two (and obviously, the doors unlocked). A few years later (but before I got a trickle charger for it), I left it locked inside my garage for 2-1/2 months without fully discharging the battery (although it was noticeably weak when I finally did start it). So, I put '2 + 2' together and figured that the key in the ignition kept the vehicle 'awake' until the battery drew down to nearly dead. Finally, I got smart about a vehicle I wanted to keep -- but wasn't driving regularly -- and purchased a trickle charger (a Battery MINDer) to keep the battery strong. Never a battery charge problem since, although when I use one on my X3, I've noted a strange characteristic: the Battery MINDer frequently goes in and out of it's maintenance mode. Still haven't really solved that one, but the X3 battery is always strong when I go to start it...

All that being said, almost every modern vehicle will continue to draw from the battery, even when the car is sitting idle in a garage with the key out of the ignition and the doors locked. Most manufacturers do what they can to minimize this draw, but devices like security systems need power to perform their assigned functions, so over time, they will discharge the battery if the battery is not replenished by driving the vehicle, or using a trickle charger.

X3 w/6-speed & Sport Package:
A sports sedan disguised as an SUV...
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