BMW Repair: battery flat-320i year 2001auto, trickle charger, car wont start

Question
hello Evan
car started fine last sunday.drove for about two miles. came back to the car after an hour and the car wont start. i got three clicks and thats it.i thought i couldnt be the battery because the central locking opened without any problems.
got the breakdown guys in the the guy put a jumper on it. car started straight away.the guy told me the battery is completely flat and the alternator might not be charging it.
the alternator was changed by main dealers just over a year ago.
anyway i drove home and left the car on for an hour. seem to be working fine now.car normally does 12 miles a day.6 miles to work and 6 miles back. is that enough to keep charging the battery?
its been fine for the past 5 days but i dont put the alarm on for fear it might run the battery down againg.
what could have caused this.i had a mechanic test the battery and alternator and he saids i need a new battery and alternator.if so why is the car starting now.
thanks

Answer
Kofi, I would replace the battery if it is an original battery. Check the year stamp on the negative battery post. Car should be okay with a drive of 6 miles per start. It is hard on the battery driving that little, but I would take it out for a more extended drive whenever possible (weekends). Test the Alternator only with a Good, Fully charged battery. You could also buy a trickle charger to hook up to at home with an auto shut off.
-Evan