I recently bought a BMW Z3, so here's some piccies, thoughts and random experiences with the car...updated frequently.
Battery Discharge Problems
A couple of months ago, the Z3 battery started going flat. At first, it was once every 3 weeks or so, so I just presumed I'd left something on. Literally, it was a case of jump starting it in the morning, and no problems for another 3 weeks.
Gradually, it got worse and worse - from 3 weeks, to 2 weeks, eventually leading me to where I am now - every day. In fact, more frequently than that; if I go for an extended shopping session, I'm only able to start it on the key 50% of the time.
Things have go so bad, I now drive around with a booster pack in the boot. And I have to use it both in the morning, and on my way home from work every day.
My dad managed to convince me it was the alternator, so we made plans to remove it and take it to a local refurb shop. I also went and bought a new battery at this point, cos the old one was beyond redemption by this time.
So, there we were in my dad's shop. We put the new battery on, and it started fine. He said that it might just be the old battery that was dead and thus not holding a charge properly. It was an original battery, afterall...and thus 10 years old. Plus, it was an easy fix - I was happy to try it.
I got 2 weeks of flawless motoring. Then it started again. I did some googling, and saw a suggestion that it might be the interior boot light not extinguishing when the boot was closed. I laughed, mainly because the post said you could check this by closing the boot with someone inside - this is a boot that barely fits 3 bags of shopping...who the hell would fit in there? Anyway, as we were putting the alternator back on (we'd removed it when the new battery started going flat - it turned out to be fine), I noticed the boot light trigger plungers on the boot, and pressed them - the light stayed on, even when the triggers were in the 'closed boot' position. So, we disconnected the bootlight.
The car needed jumping at this point - the new battery was still flat. But after that, it worked fine. One quick drive later, it had charged enough to stop/start, and was fine the next morning, and the morning after....and every morning after that for at least a week. I seriously thought we'd sorted it.
Then it started again.
So, I'm at a loss currently. Something in the car is definitely discharging the battery. I know it's not the battery itself, because when I physically disconnect the leads after a drive, and reconnect them in the morning (when it would normally be flat), there's enough juice to start the car without a boost. So, it's definitely charging, and it's definitely holding that charge.
my next possible is the FSU (final stage unit) - it seems like a common problem for discharge probs on BMWs.
My more short term goal, however, will be to fully charge the battery and put it back on - it seems I'll get at least a week or two of troublefree motoring if I do that!