15 years is good! On a W123 id attribute it to ultra low parasitic draw - only a quartz clock on many to draw from a big group 49 battery. Means it stays at a higher soc for longer.

In a w210 Id attribute it to living under the rear seat - heat is the biggest killer of batteries because it accelerates self discharge and side reactions.

A desulfation system that is permanently attached to the battery is a parasitic load. I do prefer temperature compensated maintenance chargers that offer a desulfating pulse.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)