Something to consider:

Is there a draw on the battery that you are unaware of??

Ie shorted wire etc


Have you tried jump starting the car and putting a VOLT METER across the battery terminal to see if the battery is charging?

Another way to see this is happening to start the car via jumping let it run for a while, shut it off, then try starting it agian.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)