It could be that the plates are shorting similar to what can happen with a Lithium battery since the plates are closer and the electrolyte isn't moving a path between plates gets formed.

What does your mechanic nephew say about how this fails, is it maybe brand specific ?? Has it been known and corrected in newer batteries ?

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)