similar thing happened to me, accept I ended up replacing the altenator, and after that I still had a battery drain. The shop I originally brought it to had no idea what it was (it was a meineke - yah i know bad idea but closest to my hosue at the time and i didnt have an indie).

The indie i eventually got referred too (south FL sorry) found the issue in 5 minutes. It ended up being the AC blower motor. Maybe it is for you, only thing I can think of is that this must be common on bmw if he knew what it was right away.. or had a guess, and was right.

cost me about $200 for part and labor. well worth it. Until then I was doing what you're doing, trickle charging - and disconnecting my battery when i parked. I was scared each time i got out my car that it would die by the time i got back. I basically got it down to a science, I knew that if i left it off for 4hrs or more it would be dead when i returned.

Also, if you notice that when u get a new battery, it takes 8+hrs to die, then slowly it takes less time. you probably have a drain.. that and your battery is getting weaker. but you may not let this go on for a week like I did..

i feel your pain..

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2005 LeMans Blue e53 4.8is
MODs
///LeMans Blue RBs///Intravee II///Sirius///ACS Spoiler///AiB v3///HID Fogs///LED Tails///
///215 20" Ferric Gray///MTEC Pedals///DVD Headrests///SMG M Wheel///AFE CAI Stage 2