Battery discharged what should I do?

[Ron07x5]

I have a 2007 x5 4.8l, it has a 2 year old battery installed by BMW.
About 3 to 4 months ago I disconnected the battery for about 48 hours as I worked on replacing the dreaded oil thermostat gasket.
I reconnected without "introducing the battery" again, did I need to do that? What tool should I get to introduce the battery if needed?

This happened ... Yesterday my wife noted that the x5 had it's date and time cleared.
Then today as we were driving we had the AC fan slow even through
is was on high. Then 10 minutes after that a clunk is heard and a warning that there was transmission failure. I pull over into a parking lot and then
could start the car the symptom seemed to be a discharged battery.
I had the x5 towed home and put a battery charge on it and now the
transmission and all seems fine etc.

So .... talked to BMW friend and he thinks the car lost it's battery registration and further he seems positive the alternator will be fried and advises I only buy a vin matched bmw alternator at $750?

So what do you all think I should do?