From BMW 'Battery Basics'.
Quote:
Charging and trickle charging of the battery
...If the battery is charged when fitted, the charging process must take place via the jump start terminal point. This is the only way to ensure that the charging process is detected correctly by the vehicle electronics on vehicles with an intelligent battery sensor (IBS). If the battery is charged directly at the battery terminals, misinterpretations of the battery condition and, under certain circumstances, unwanted Check Control messages or fault code memory entries can occur. The cigarette lighter is supplied with voltage by the front power distribution box via switched terminal 30B. After terminal 30B off, the relay de-energizes. This means that a trickle charger connected at the cigarette lighter would be disconnected from the battery. Only charge the battery via the jump start terminal point.
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Most user manuals also contain a warning not to use the 12V sockets.