I would expect 12.5-12.8 volts for the battery itself, and in the upper 13 range with it charging. Not sure what the official BMW figure is.

Your battery could just be low, and your alternator could be the problem.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue