One person had their battery replaced under warranty got an Exide battery 90AH BMW P/N 61-21-7-553-954. The replacement explanation was "Replaced as per ISID2 Production Error".

getbmwparts.com lists the cost as list price $299 and their price as $239.20. It also says that it is an AGM battery. They don't say who makes it.

AGM is "A newer type of sealed battery uses "Absorbed Glass Mats", or AGM between the plates. This is a very fine fiber Boron-Silicate glass mat. These type of batteries have all the advantages of gelled, but can take much more abuse. We sell the Concorde (and Lifeline, made by Concorde) AGM batteries. These are also called "starved electrolyte", as the mat is about 95% saturated rather than fully soaked. That also means that they will not leak acid even if broken."

Chuck