I bought the small battery at Advance auto parts, about $80. it was not a problem to install. It's underneath the climate control air filter on the passenger side of the firewall.
The scary red warning screen is gone.
Both batteries are dealer list price $400 at the parts counter.

I want to find out who makes the batteries Mercedes-Benz installs in the vehicles made in Alabama; I bet they aren't imported from Germany. After all, if Germans are driving one of the sport-utes, it is made in USA.
Other than having the Star on them, I don't think the OEM-marked batteries are worth twice the cost of aftermarket batteries.

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Providing a home for these cars:
1951 Buick Special De Luxe
1977 Rolls-Royce Silver Wraith II
1970 Rolls-Royce Silver Shadow
1986 Mercedes-Benz 420 SEL
2005 Mercedes-Benz E 320 CDI
2012 GMC Sierra 1500 4X4
1927 Pierce-Arrow Series 80
1931 Pierce-Arrow Series 43
1926 Ford Model T coupe