At 5 years, I'm going to replace my main and auxiliary batteries.

For the main battery, I ended up going with the Bosch Platinum Group 49/H8/L5 AGM. Like that it has a 4 year warranty. Got it at Pep Boys - they are currently running a special for 20% off, plus another $10 mail in rebate - with next day in store pickup. Total cost after tax and bogus "core charge" is about $190 here in California.

I bought a relatively cheap auxiliary battery on-line from Interstate batteries. 12V 1.3AH AGM - Part #SLA105. Total $22 including shipping.

Below is a link for the main batter. Enter code PEPBATT20 at checkout for 20% off... Go back later and download the $10 rebate coupon and mail in.

https://www.pepboys.com/product/deta...203.0L%2520Dsl

Plan to install both this weekend...

Went in and picked up the new battery yesterday - the guy at the counter wiped off the $15 core charge just for the asking, so the deal got even better.

Took about an hour to install both the new main and auxiliary batteries this afternoon. Very straightforward following one of the videos of the process posted on youtube. Hardest part was figuring out how to get the old auxiliary battery out - i just could not figure out how to open up or remove the cage that holds it in place. I hooked up a battery tender under the hood while I was replacing the batteries and as such didn't have to reset any computers or other settings once I was done.

Still seems very strange the vaunted Mercedes engineers decided to set this up so you have to slice up your carpet to get the old battery out.

All in all not a very difficult project - but still 10X harder than replacing batteries that are in normal locations in the engine bay or trunk.

Thanks for the write up - perfect timing. Completely dead battery yesterday. I saw the exact same battery today and want to try it, but am concerned about fit. OE says group size H8 but this one is L5. Can't find anything to confirm size requirements. Did you have any issues with the bracket fit or movement in the bay? Also, can you only get aux battery online? Might as well change both while I'm in there and if they have it at Pep Boys I'll grab one. Thanks again.

Pics?

Went with Bosch battery at Pep Boys and found an auxiliary at Batteries + Bulbs. Both discounted so the entire project was a win on parts and certainly labor. Watch the video from MB Autohaus and you'll be good to go. Note the pry bar is essential and you'd be doing yourself a big favor having the magnetic telescope for the nuts and mounting bracket. I'll also note that I connected a trickle charger to the posts under the hood to make sure the electronics didn't lose power and need a dealer reset. No idea if that's true, but I took no chances. I also kept the key fob out of range of keyless entry and the front driver'a door open the entire time as I saw someone else mention that doing so completely shuts down the COMAND and other electronics further reducing the chance of a reset. Again, no idea if that's true but just a precaution. Finally, I saw someone say in all CAPS to never, never touch the positive terminal wire to a grounded metal surface during this procedure so I was super careful to avoid that, as well.

Extra tip: If you're also replacing the auxiliary battery, the factory installed auxiliary battery has the terminals on the inside of the docking cage. It's tough to get in there and get the leads off and even tougher to press the connection tabs on the battery down far enough to get them under the top of the cage (meaning they get caught and you can't pull it out). After about 20 minutes of effort to push them down (hand and tools) I ended up muscling the battery out which catches the tabs on the cage and pushes them back the other way. It closes the loop and sparks a tiny bit, but I got it out and reconnected the leads on the new battery with the tabs facing out toward the battery bay via a tip from a tech on another blog post. Worked fine and the wires stayed clear of the big battery.

The entire project probably took me 1:20 and 20-30 were spent wrestling the auxiliary battery. I went slow as it was the first time I've done this and it paid off. Saved a bunch of cash and learned a thing or two. I'd say if you're even somewhat handy that once you get in there it's easy to moderately difficult and just takes some patience. REMEMBER: Always disconnect the negative terminal first when pulling the battery out and reconnect the positive terminal first when putting the new battery in. No idea why, just following instructions I saw 5-6 times while researching.

Where exactly is the aux battery? I changed my main battery a couple years ago after Sears refused! Pretty straight forward after watching the youtube video. But I didn't think to look for an aux battery.
What happens if the aux battery starts to fail as long as the main battery is good?

Edit: OK, found it in another forum. See Post 24
https://mbworld.org/forums/gl-class-...ry-issues.html
However, looks tough to change without also pulling out the main battery.
Guess I'm gonna take my chances and hope it lasts another 2-3 years until I need another new main battery.

Any options for more than 850CCA?

Yeah, there's no way you could change it without taking the crank battery out. It's the small, black plastic box in the upper-rear right of the battery bay. And I yanked on it to bend the terminal tabs back to get it through the mounting cage. Pushing them down just wasn't happening. Probably 5" x 3" x 3" and is only 1.2Ah. I've seen people say it's for the shifter only when the crank dies so you can get it into neutral and onto a flatbed, Others say it's to keep the COMAND from resetting. Who knows. Use a trickle on the engine bay posts when you're working and it won't matter.

Just a side note/suggestion on replacing the battery(ies). The passenger seat can be removed in about 5 minutes. Without removing the seat belt, just spin the seat out of the car and set it down facing backwards. It's actually MUCH easier than it might sound and is worth the extra few minutes to do so.