I am not familiar with the E53 charging system, but I would expect to usually see 13.8 volts when the engine is running on most vehicles.

My E70 runs about 14.3-14.8 volts when the engine is running, but the E70 uses an AGM battery as OE, and they typically like a higher voltage, e.g., a couple of tenths or more, as compared to the older flooded-cell batteries.