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Join Date: Jun 2014
Vehicle: '06 SLK55
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How I chose the new battery for my SLK
If you have been following my adventures with my '06 '55 (at least the wife says it is an adventure every time she drives), you know I have been chasing a mystery intermittent electrical issue with my top and passenger window. A week or two ago I discovered that the prior owner had installed a smaller than spec battery and it was pretty dated. So began my battery quest.
Attached is a fairly "exhaustive" list of specs for the OE and aftermarket batteries specified for the r171 & available stateside. As it turns out there are only three major manufacturers for auto batteries in the world, Exide, East Penn/Deka, and the largest, Johnson Controls ((JCI) who makes the OE under the brand Varta).
Everything out there is just a sticker on a battery made by these big three. An honorable mention goes to the smaller company Enersys, the maker of the Optima battery. They are one of the original manufacturers of the Absorbent Glass Mat (AGM) battery technology about 50 years ago. But more about that in a moment.
As I posted in another thread, the ’05 SLK was equipped with a Group 49 (H7 in euro spec) battery that is larger in capacity and size as compared to the ’06-’11 battery. In ’06, MB reduced the available battery tray area and went to a smaller Group 98r/H6 AGM battery. For the remaining vehicle production the battery remained roughly the same size and capacity with Group 48/H6 but MB cut manufacturing expenses further, this time by removing the AGM specification. A non-AGM is likely what you get today when you buy the factory replacement battery for $165 or so.
So what is AGM and is it worth the extra cost? Basically an AGM battery, as opposed to a standard flooded cell battery or a gel cell, is made with a very fine fiberglass mat inside that absorbs the acid. This makes the battery spill, leak proof, and virtually vibration proof. Originally designed for military use, the design has the benefit of delivering higher current on demand and it can also be discharged deeper and more often than a flooded cell while still keeping a comparable life span. Some even say they also last longer, evidenced by longer non-prorated replacement warranty times.
Did you hear “probably better to use if you have a SmartTop” in that description?
You can Google to find out more detail on the design, but most performance car owners on their respective forums also seem to lean towards AGM.
That said, I have also included several flooded cell batteries on the spreadsheet just because they were easily recognizable while I was researching all this. Most, with the exception of the MB sourced non-AGM battery, are priced about 25% less than the average AGM battery (except the budget Wal-Mart Group 48 @ $119.95), so choosing an upgrade to AGM is a pretty easy decision.
Speaking of making a decision, with over 20 batteries to choose from I pretty much went with full replacement warranty period instead of CCA output to narrow down my decision. All AGM outputs were relatively similar, but even choosing by warranty period threw a bit of a curve at me. The three direct manufacturers all specify a 30 month full replacement warranty.
But I was told by the rep for NAPA, a multi state national distributor for East Penn/Deka that battery distributors can shorten and extend the manufacturer’s warranty at will. So for the same Group 48 AGM battery, in the case of NAPA they will honor only 18 months of the East Penn/Deka warranty leaving you out in the cold for the remaining 12 months. On the other hand Batteries Plus (another nationwide distributor) will warranty the same East Penn/Deka battery for 48 months by adding an additional 18 months themselves to what East Penn offers.
Considering I’m in a hot climate here in S.E. Florida I opted to restrict my choices to batteries with the four year warranty. So that ruled out the OE battery for me, even though they would come to change it for free, since MB would only offer me two years. Keeping my decision difficult, that left four batteries left to choose from; two from East Penn, one from Enersys, and one from JCI.
The JCI (Interstate) non-AGM battery was rated by consumer reports as the best Group 48 battery of the four they tested. For some reason they did not test a Group 48 AGM. But I chose to stick with AGM anyhow and also decided to stay under the $200 mark. So the $259 Interstate AGM was out. That also ruled out the Enersys Oddesy, probably the best of the bunch. Milspec AGM is just about all Enersys does. But at over $320, they can keep the juice. I can use the extra dough to put towards some carbon for my dash.
So I went with the 48 month Bosch AGM by East Penn since Pep Boys had a 10% off on line coupon and the battery comes with a nice big German looking Bosch sticker…
So at the end of the day, a cool sticker is what closed the deal. Nothing like making a scientific decision.
Last edited by Lioninstreet; 02-18-2015 at 08:41 AM.