there is a big relay somewhere in the system too. it also can cause this havoc. Its supposed to make the 2 batteries independant of each other. I read about this a long time ago when these cars were pretty new and this problem was encountered by a person in UK.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)