John Corneille

D49 can you tell me the FIAMM part number please? I could only find the genuine MB part at nealy US$400 when I replaced my auxillary battery 18 months ago.....and it had died again during covid lockdown!

jmattioni

The main battery does the re-start for ECO. During the re-start the Aux battery is switched in to take on all other loads. This prevents a voltage sag that might, for example, cause the radio to drop out during the re-start or the lights to dim.

The Aux battery also powers the ignition switch module and the transmission gear selector module. This enables you to get into the car and shift the trans into neutral with a totally dead main battery. Seems like an important function in a bad situation. I would definitely not remove the Aux battery to get rid of ECO stop/start.

Jspann07

Thanks for this post, started getting the auxiliary battery malfunction notice and then the distronic plus quit working. Ordered a battery from FCP, currently on clearance, delivered to house for $108. Paid close attention to your detailed instruction and in 40 minutes had it done and cruise control works again. I will add, with top down, passenger seat all the way forward, belt removed, towel over side of car, I went at it. I did not have to remove any additional trim other than the two pieces noted. Battery cage came out putting it over that bottom left tab and pushing on the console trim while moving cage to the left. Saved $330

rmorin49

Good info. Nice to know owners can install the auxiliary battery with normal tools at home. What is the age and mileage of your SL?
Thanks for your post.

Jspann07

My car is a 2015 and has 35,000 miles, came from up north and the main battery had already been replaced. Guess the cold got to them.

rmorin49

Mine is also a 2015 but was a Florida car. Don't know if either of the batteries was replaced as part of the CPO certification process. I have been using a CTEK on each battery if I park the car for more than about 3 days. Have never seen any warnings about either battery so far.

Dr. Manhattan

To those of you who have accessed their aux batteries, was there anything holding the battery down? Mine seemed like it was just sitting there loose with nothing holding it down...

Dr. Manhattan

My car is a 2017, but here's the number on mine: A 000 982 70 08. The label with the number on it says both FIAMM and M-B, so this may just be a M-B part number. If so, I'm sorry I couldn't help you.

Dr. Manhattan

Wouldn't connecting a CTek or other trickle charger under the hood do the trick?

hornethandler

Most likely, but I was hoping to find out what Putter used because we know it worked.

I just checked and my CTEK MSX 5.0 charger only supplies charging current when connected to a voltage source (battery) that is not below the charger’s threshold voltage (typically 8 to 11 volts). So, I don’t believe it will substitute as a voltage source when swapping batteries.

hornethandler

I just checked and my CTEK MSX 5.0 charger only supplies charging current when connected to a voltage source (battery) that is not below the charger’s threshold voltage (typically 8 to 11 volts). So, I don’t believe it will substitute as a voltage source when swapping batteries.

Dr. Manhattan

Apparently, in this instance the CTek's smart features will out-clever themselves and render it useless. I do have an old stone axe 1A Schumacher that I bet would do the trick if everything else in the car was shut off...

hornethandler

The “adults” decided some time ago to design that particular safety feature into most battery chargers.

The first time I came across it (many, many years ago) I thought the charger was bad. My uncle, an aircraft carrier electrician at the time, told me about that feature. I’d forgotten about it until after I posted above that the CTEK might work, so I humbly apologize.

Dr. Manhattan

No worries. Thanks to you, I've added my old 1 Amp charger to my list of things that need to be brought to GA from WI the next time I make the trip in that direction.

I just checked the manual for my CTEK MUS 4.3 and it says it's usable all the way down to 2.0V, which I know still won't work as we're talking about 0 volts while swapping batteries. I have to think that your MSX 5.0 would probably have the same lower limit...

TyroneT

Didn't rive my 14 SL550 for a while.....got caught out with a dead battery and easily jumped.
Few weeks later......dead again......

Read all the posts with the main and aux battery.......ordered a new Aux battery ...OEM for $99. After arrival, removed panels etc just behind passenger seat.....anyone telling you it is difficult, or need to break anything to access the battery is in inept ape. And anyone willing to pay the dealer hundreds to do the 30-45 minute job deserves getting gouged. Point being....really easy to do, might have to fiddle with large plastic cage directly under the panels, but not a big deal.....at all. Hook up is simple and obvious....
Once hooked up, car still didn't start. (as I assumed the starter used the primary battery, contrary to some posts)...so, also had the standard battery which is mounted in the trunk.....super easy to access and switch.....

All in, maybe an hour of work......no gouging dealer price....no "reset" issues......no "programming" bs....or keeping memory on while doing swap......everything works perfect......
DO IT YOURSELF!

Learned the Mercedes gouging lesson years ago with my 04 CL500.....had a rear wheel bearing go out. Bought OEM replacement bearings for $50....wasn't sure the bearings were the problem so went to dealer to diagnose.......they confirmed the bearings, then told me that their "cost" on just the bearings (parts) was around $200 (do tell).....and of course, labor would be a significant addition (Westlake Village, Ca.). Same dealer wanted around you $500 for new remote key with programming......to be fair....the dealerships in Calabasas and Van Nuys wanted the same.....lucky for me, the Downtown LA dealer was half the price.....so, if you must go to a dealer, shop around, they all gouge at different levels. BUT.....do the battery yourself, just because we drive a Mercedes, doesn't mean we have to be totally inept.

jmattioni

The main battery (in trunk) on the R231 is very easy to replace. The main issues I encountered were (1) it's weight (watch your back!), and (2) the vent on the replacement battery (a Diehard, Advanced Auto) didn't come with the nipple for the vent hose and the one in the original battery was apparently glued in. I found a suitable part to deal with that.

I used a computer UPS battery on the jump terminals to hold the system. Ignition off and key fob out of range to minimize load.

My SL is a 2013 and STILL has the original Aux battery. No issues.

George Stancil

My 2015 needed the auxiliary battery. I bought one and put it in. At that time I installed a trickle charger. I let the electrical plug extend out from the base of the box the battery is in. Now, I just plug it into an extension cord when it will be sitting for a while. If you do that, be sure to get one that doesn't have the little red light that will actually draw the battery down when not plugged in.
I realized I had a battery problem when I had a some distronic malfunctions.

Dr. Manhattan

The aux battery shouldn't have any effect on Distronic functions as it isn't connected to the main buss at all unless the starter motor is engaged during the eco start/stop function. Aux battery is only permanently connected to the keyless entry/go and transmission (shifter) modules. I'm wondering how a new battery solved your Distronic problems. Interesting...

George Stancil

Hmm, me too. But as I remember I was having trouble with the adaptive cruise control holding, It seems as I did some research and decided it was that battery. After the new one was installed, everything has worked.
But I am not mechanic, and do not do diagnostics , just enjoy cars. I did not intentionally mislead, just confused as time passed I guess. Obviously, if they don't communicate there is no relationship unless there is some shared sequencing or function order some where, but that's way beyond me.

George Stancil

I did find some information that states the aux. battery controls convenience features, nonessential things such as heated seats. So maybe it is possible that adaptive cruise is one of the nonessential items,
I also saw this thread
https://mbworld.org/forums/w212-amg/...operative.html
where other people, but different models had success fixing AdaptiveCC by replacing the aux. battery.
And this also is an interesting read
http://www.mercedesmedic.com/battery...ning-solution/

Box5232

Mine does have the hold down, but I found that the new battery terminals are closer to the edge of the battery, and I had to cut a little off the battery hold down. I used my dremel, and it was very easy to do. Total time, not including going to MB dealership for battery, about 45 very easy minutes. Cost $145 for the battery. Someone probably replaced the battery themselves and could not get the hold down back in place and ditched it.

SLLover

After needing a couple of jump starts I spent a few hours online trying to get a definitive explanation of how the two batteries work, what they power and how to replace them. It was not spelled out in black and white like I am used to finding when searching for how things work. The auxiliary battery operates the traffic stop-start (not the starter motor), hill-hold and some other minor functions. It is necessary for proper operation of the car and also serves as a sort of capacitor when starter cranking amps are pulled from the main battery in the trunk to keep other electronics functioning properly. The primary battery in the trunk powers most of the "consumer" electronics as well as the starter motor. When I ascertained that I knew what I was doing I went to O'Reilly's and got the premium 49 group AGM battery for $200 and swapped it out. Perfect fit with overflow ports and an instant fix and with no need to supply a proxy charge to the car; everything fired back up. I also wanted to access the auxiliary battery as I am a curious sort and wanted to be able to access it for future reference if it needed replacing. These are basically motorcycle batteries to supplement the main battery and power a few functions. A previous poster stated correctly that the "cage" surrounding this battery can be removed after pulling the front and top plastic panels from the passenger side rear of seat bulkhead. He said that you need to push the center console trim away from the cage (firmly) as you pull it out to clear the two ridiculous tabs under the console trim. (The tabs do not provide any support that I could see.)That is correct but the tidbit that was missing was that the lower left cage bolt tab has quite a bit of give allowing you flex it forward as you pull the cage up and forward to clear the rear bulkhead and side console trim. I am going to use a dremel to remove the useless side tabs for ease of replacing the auxiliary battery in the future. I hope this helps some of you that are faced with the dilemma of paying a dealer $800 for a battery replacement.

rmorin49

Thanks for this update SLLover. I have been using a CTEK unit on both of my batteries whenever my SL is not driven for a week or so in an effort to extend the life of both batteries. I discovered that the previous owner had installed a CTEK pigtail on the auxiliary battery when I opened up the panel. I leave the pigtail exposed now for easy hookup.

crobb65

I first encountered the brake hold function in 1978...on a Jaguar 3.8S sedan.

jimbotek

I started to do this same job and then decided to take it to dealer, cost ~$200.