Have you run an energy diagnosis on the battery through DIS? I just changed my battery over the weekend as it was starting to,give out a lot more frequently. I was trickle charging it when I got home at the weekend every now & again but the short journeys to work was killing it.
If you want to do the energy diagnosis just give me a shout, I can be found on Dublin's south side sometimes!

http://www.bmwhaus.ie/forum/showthread.php?tid=142&pid=30872#pid30872