Mercedes-Benz SLK Battery

Sporty (sportlich), lightweight (leicht) and short (kurz) is how the Mercedes-Benz SLK derived the name it bore from 1996 through the 2020 model. Throughout this run, the SLK roadsters used the same size battery, the Group 48 battery (aka 98R, H6 or in the case of OPTIMA DH6) for nearly the entire duration of production, with a few notable exceptions. The BCI fitment details on these cars can be a bit of an eye chart, so we'll help you sort through what all the numbers mean. First, we'll deal with the footnote numbers that appear in the second column.

Many years of SLK production include some combination of the footnote numbers of 54, 66 and 69. The 54 footnote simply indicates the battery is vented to the outside atmosphere. That's fairly standard and all OPTIMA DH6 batteries come with provisions for externally venting the battery. The 66 footnote indicates the Cold Cranking Amps (CCA) measurement is a European version of that number, which can vary slightly when compared to US CCA ratings. However, OPTIMA batteries are typically conservative in what they are rated at for cold cranking amps and our DH6 YELLOWTOP conservatively-rated at 800 CCA would have no trouble meeting either standard. The 69 footnote indicates the battery is required to be an AGM battery, which is also not a problem for the OPTIMA DH6 YELLOWTOP, as all OPTIMA batteries are AGM batteries.

The third column indicates the battery group size, for which we will provide conversions here:

Some model years also had a small, 12Ah Group 400 auxiliary battery, for which OPTIMA does not currently offer a direct-fit replacement. You'll also notice the current BCI catalog only goes back 20 years, but the fitment tool on our website covers the entire production run, in case you drive a 1999 SLK, like the one pictured above.

As you can see from the BCI chart, the first exception on the DH6 fitment would be the 2005-2011 SLK350, which were sometimes equipped with an H5 or Group 47 battery from the factory. However, the H6 or Group 48 battery was also listed an option in those years and OPTIMA offers a direct-fit DH6 battery for those SLK350s.

The other notable exception for battery fitment for the SLK roadsters came in the 2006 SLK55 AMG, which used a Group 49 or H8 battery in the 2005 model year. While OPTIMA does not make a direct-fit replacement battery for this particular application, our friends at Weistec have a solution in the form of their H6 to H8 adapter, which will allow you to use a smaller, more lightweight DH6 YELLOWTOP in that vehicle.

One final thing we want to mention is that battery performance and lifespan is maximized when voltage is properly-maintained. The OPTIMA DH6 YELLOWTOP is considered fully-charged at about 13.0-13.2 volts. Whenever any battery is discharged below 12.4 volts and is left sitting in that state, sulfation begins forming in the plates, which diminishes both capacity and lifespan. For that reason, we recommend keeping the battery in your SLK roadster topped off whenever possible with a quality battery maintenance device. We're so confident this will improve performance and lifespan, we will extend the free replacement warranty on your OPTIMA battery by a full year, if you purchase it directly from us at the same time as purchasing an OPTIMA Digital 1200 charger.