Do you have a scanner that will pull the battery level histogram? It's a very good way to get a sense of is the problem more related to the battery or the starter.

Typical starter failure presents with the symptom you describe. Just takes longer to start. A defect in the starter will cause it to take much more current. It drops the voltage from the battery getting less torque at the starter, lower RPM.

Get a current reading on the starter cable and a load test on the battery

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)