Alternator or you got a short in your system somewhere.
Not sure if you can do this with computer controlled bimmers, but with older cars you could simply removed the + led with the key off, then place it back on the terminal. If it made a spark then you either have a short in your system or an electrical device that is stuck on drawing on your battery.
Im sure there is a more scientific approach to testing your system for a draw or short.
1996- BMW 328IS
<img src='http://img.photobucket.com/albums/v242/gws226/TEX/neverleanedsig.jpg' border='0' alt='user posted image' />
Performance: Zimmerman Cross Drilled Rotors-- PBR Deluxes-- Cosmos CAI---- Turner Pulley Kit--- Fan Delete mod---
Beauty: 18 polished M-Parallel replicas, Inpro Euros w/ Enchanced Angel eye kit, 20% tint
Clear corners/sidemarkers ---- Eurospec (Depos)
To do list:
UUC System U TDM- 3/05
M50 manifold converstion 3/05
off to the tuning shop for custom software 4/05
Cams/lightened flywheel, throttlebody........ well lets see what happens