The standard battery is an absorbed glass mat type (AGM). It is to be "registered" with a scan tool. If this is not done the vehicle can adjust to the new battery on its own. When the battery registration procedure is not performed several effects are seen; they include trouble codes in modules or the system can be put into "transport mode" in which whole systems can be disabled. The most common is that the hvac system is disabled. A battery saver should not be used on these vehicles as it will most likely blow the fuse for module memories.
BMW is trying to use this system to predict when the battery reaches near the end of its useful life. It is called the "intelligent battery system". A regular lead acid battery can be installed but you must tell the computer that you switched types. In more recent years they have improved the system further; it now requires a genuine BMW battery and they removed the switch to lead acid option at that time.