I would have thought an AGM battery is pretty short-proof but maybe not. If voltage levels off then that is where charging levels off, it doesn't "keep charging." There could be a problem which hasn't shown up yet and it would have been nice to have a voltage reading on the charging battery before you took it out but if the voltage was over charging then I would expect the at rest voltage on the original battery would have been higher.

I don't think it makes that much difference but was the battery logged in as an AGM when it was replaced?

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