It's a new one on me. One thing that was odd is the car would not remotely try to start up until we replaced that extra battery, which is referred to as "the consumer battery" in some of my reading. Like I said, the main battery's voltage was like 12.6. Whereas the old battery in the trunk read zero. Maybe it powers all that is normally parasitic draw.

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Te futueo et caballum tuum

1986 300SDL, 362K
1984 300D, 138K