Thanks for all your replies.
I was aware that original batteries that come with domestics usually last 5 yrs.
You are correct that factors
play into lifespan. Climate here is temperate.
The five years service so far
sits o.k. with me now although
I expected more from an OEM
Benz battery.
No problemo.
FYI for the readers : the C280
has the battery in the trunk.

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1979 300D
133K miles
1989 300SE
1995 C280