I have had conversations with the instructors at BMW school about this issue and at first it does seem that the process should be satisfied as you have pointed out. However, the physics of the AGM batteries is such that from an engineering standpoint, the actual time to bring the charge level back to full charge is more than just a few moments. Also please consider that these cars do not stop consuming power just because you have shut the car off and locked the doors. BMW's can take up to 16 minutes to go to " sleep" and even after they do, there is still a key off draw of about 50 ma. To continue communications in the CAN BUS system. So at the end of the day the time to recharge the battery to full charge might be 20 minutes. Hope this clears this issue up for you.
July, 2, 2008 AT 1:42 PM
Thanks for the quick response. Is this to say that a BMW must be driven 20 minutes every day?
July, 2, 2008 AT 2:58 PM
Is there a problem with that? : -)
July, 3, 2008 AT 8:48 AM
Not everyone commutes an hour to work each day. What about shopping with repeated restarts or if you left the car for 3-4 days? Would that mean that you would have to recharge for 1-2 hours?
July, 3, 2008 AT 10:19 AM
Typically what it means is that after several weeks or months with this " pattern" of operation the battery can still start the car but it does so with diminished capacity and can shorten the life of the battery. Many times I have gotten a (brand new or just delivered) towed in vehicle that had insufficient charge on the battery to start the car. It also has a tendency to set erroneous faults in the car due to the HUGE dependency the new BMW cars have on a strong battery.