You can use the Hidden OBC Menu, http://www.scoopz.com/m5board/E60_Hi...structions.pdf

Proper charging Voltage is typically between 13.5-14.5 Volts.

Also you can use a Voltmeter as well, but best to leave this on the vehicle for a long time.

Something like this may also be helpful - http://www.amazon.com/Temperature-Mu...arette+lighter

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2008 4.8i Black with Tabacco. Black Y-spoke wheels and shadowline trim.