Question on battery replacement

Hey guys,

Wife's 2008 328i has been having a problem for a few days where the clock gets reset (and displays "--:--" instead of the time) when the car has been turned off for some time. Doesn't happen all the time and sometimes the clock falls behind instead of getting reset.

Took it to my mechanic (not dealer) and he said it's because the battery is dying. That makes sense and I had guessed as much (even though there are no problems starting and the memory settings for seats, radio etc. are fine). But the odd thing is he said I need to take the car to the dealer to replace the battery. Apparently there's some calibration that needs to be done or something of the sort that can only be done at the dealer. He said a generic battery installed by someone other than the dealer will be fine for a short time but I'll start to see electronics issues like this pop up again after that.

Anyone else run into this? What's so special about BMW batteries and electronics systems that can't deal with the battery being replaced by someone other than the dealer?