At dealer(retail service center) we see well maintained batteries going 5-6 years, but 3-4 is the norm. Replacements are DOMESTIC made & go about the same with regular service. Underhood batteries don't last as long.

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MERCEDES BENZ MASTER GUILD TECHNICIAN (6 TIMES)
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. Shopforeman
190E 2.3 16V ITS SCCA racecar