In browsing that forum, I found a topic where they were discussing the high price of a battery replacement in a BMW...quoting dealer prices of up to $500.
Seems if you replace a old battery with not of the same type or with dissimilar enough specs, the car needs to be reprogrammed to charge the new battery correctly.
They said that this work can be done cheaper by independent garages, but unless you have the computer gear it was not a DIY job.
Also, if you replaced it with the a OEM equivalent battery the programming required a less expensive "computer" that might be DIY if you had it.
Just sounds WAY too complicated, but then I have not purchased a new vehicle since 2000.