Hi, based on the appearance of a red warning on the instrument panel, I am looking at replacing the battery on my car. According to the owner's manual there are 2 batteries. Should they both be replaced?
Dealer-wise, what am I looking at to get OEM batteries?
The warning came up when the cold weather hit; the car cranks a little slowly, but starts right up and it's fine once a few miles and a warmup are done.
Based on the age of the battery (ies) though it's probably time to replace it/them. The one in the trunk is an OEM battery; I don't know if it was ever replaced either.
Thanks.

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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

Yes W211's have two batteries. NAPA sells the correct replacements for a fraction of what the dealer charges. My dad's '05 needed them replaced at 72,000 miles.

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1981 300TD 4 speed manual
Euro bumpers, zender valance and skirts, H&R springs, billy HD's, leveled sls, real AMG Pentas 16x8 et11, vdo boost/egt gauges intergrated into ash tray, eurolights, led 3rd brake light

Read this: 06 E320 CDI 'Low Battery' Display discrepancy?

Thanks a bunch for the great info. I did read the recommended posts and they were spot-on.
The dealer list for both batteries is $400. I will get a new aux battery at the local parts store tomorrow.
As for the quality of batteries, there are only a few manufacturers, and I just replaced a Wal-Mart battery in a 420SEL, which was bought no later than 1999 by a previous owner. It was in the car when I bought it in December 2003. Dang, how much better could a battery get than that?

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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

I believe the W211 2nd battery disappeared (along with SBC brakes) in model year 07.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

OP (Original Poster
If your engine starting is dragging, then your CCAs (Cold Cranking Amps) sound like they're compromised by either a loose connection, or perhaps a bad cell, OR a weak battery because of age.....?

The auxiliary battery wouldn't create a dragging start to the engine, IMO (In My Opinion.....)

As far a replacement battery goes? I'm kinda sold on the Mercedes-Benz branded battery sold at the dealerships - since that's all I've ever replaced in my MBs owned..

If your're a member of MBCA (MercedesBenzClubOfAmerica) ($45.00 a year membership costs) show your card at the parts counter of your MB dealer for a discount off what retail is on replacement batteries. The MBCA Club Discount is worth it, if you're doing any amount of MB dealer parts buying on an annual basis....

Some swear by the MB brand batteries.

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

Stop by an Interstate independent battery store that has a good stock of their offerings. I did several months ago and found a good selection for the diesels. Their top-of-the-line model AGM is said to last up to 3 times a regular battery. These are very pricey as I recall. I used Interstate to compare pricing on what the MB dealer sells. The MB battery is less, after the MBCA Club discount is applied.