CTEK or battery tender is in your future. Also locking the car even in your closed garage assuming you have one is a must otherwise every 4 hours your computer turns on and checks the health of the entire car.
Your battery is toast plain and simple and is on life support. If you want to get caught out and not be able to start it that will happen soon expecially when a real cold snap catches you.
I have a Batteries Plus battery in my M3 I replaced in 2004 but I use a battery tender when not driving the car. My March 2007 335 is still on its OEM battery again because the battery tender prolongs its life.
Time for a battery and registration but do use a tender when you are not driving the car for more than a day and weekends if it sits. I place my battery tender inside the engine where the jumping lugs are. I just pop the hood plug it in and when I want to use it I unplug it. I have the cable installed permanently in my M3 and will do the same for the 335 where I just plug the tender connector into the connector to the jump lugs. This takes maybe 30 secs to do at most...