Yeah, that's a bit of a kludge and that is a fair bit of corrosion on the distribution connector. I would put it back to original with 1 12v Delco pro AGM. At the very minimum load test the batteries and clean up the connections.

And yes, you only have 50A hrs with both in series for 12 volts.

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