Battery test
WARNING: This page is about a different car, the 2013 BMW 550i GT, 2012 BMW 550i GT, 2011 BMW 550i GT, and 2010 BMW 550i GT. However, it is still accessible from the selected car via links, so may be relevant.
The battery acid density can be used to measure the state of charge. However this check produces uncertainty caused by a design-related range of variation. The acid density e.g. for a charged battery is 1.28 kg/l (in the variant for hot countries the acid density is 1.23 kg/l).
Another interference factor is the acid layer immediately after filling with distilled water.
Battery wear with partially sulfated and/or heavily contaminated panels will also lead to incorrect acid density test results.