Hi there. Yes this has probably already been said, but a badcell in the battery would cause your battery to read erratically at times. Other times it may start fine for a weeks straight and boom nothing when you need it. The battery should read anywhere from 12.5v-12.6v without the car running. The with it running, your reading should be anywhere from 13.8v-14.2v for a good alternator. Sometimes the Alternator can show an excessive ripple from a faulty diode. Other times it can be that a diode is open. If at all your battery light no a bunch of others come on and you start losing the radio, lights etc...your alternator has not the dust. You will have to make sure it isn't dropping when reading a voltage sufficiently, which in your case, is not the problem. Make sure the battery tests well. The maximum amperage your car is allowed before you need to get it checked out is .79 amps. Anything over that is suspect and will need to find the drain. I will say Bmw's do have a high draw for the first 5-10 minutes, but it slowly drops in normal circumstances. Besides getting it fixed, there is an alternative. It is a pain in the butt, but you could disconnect the negative(ground) to your battery and make sure it isn't making connection anywhere. This will keep the computer or nothing that is draining your battery from doing its task. It should be a 10mm and it will start when you are ready to start it. I hope this is somewhat of a help. I know what it is like to have a dead battery and need more juice to start it. Maybe I you could describe the noises it is making when you try to start it, then we could be of more assistance. I have a 900cca AAA battery in mine. I have no drain and left my parking lights on for 12hrs after working an 18hr shift and it fired up.