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Went to the store, purchased the battery for $23.84 and replaced it in not even 5 minutes. Online I see people taking the bottom panel under the dash out, etc. quoting 30 minutes to 1 hour to replace. Completely unnecessary, just take off the side panel, remove the 2 screws holding the fuse box and 1 screw holding the battery, pull it out, reaplce it, pop it back in, and done.
I got 2 batteries so I have another one in case I ever need it.
Excellent write up! Were you having issues with the battery or was this just a maintenance precaution? Also what store did you get the battery? Thanks.
Excellent write up! Were you having issues with the battery or was this just a maintenance precaution? Also what store did you get the battery? Thanks.
I would get "Auxiliary Battery Malfunction" after starting the car and it would disappear. It was likely on it's way out.
I just changed out my AUX battery. Great write-up! How did you get the battery and the bracket out? I could not fit my hand in there to get a good grip to pull it through. When I did have a hold on the battery and bracket it wouldn't come out. It didn't feel like it had much slack and I didn't want to pull on it to the point that I broke or disconnected anything that I wouldn't be able to reconnect. I was losing light fast so I just took the terminals off the old battery and attached them to the new battery and put everything back in the hole. To my surprise, there is a lot of room back there. Normally when you take these things apart all the wires and modules are crammed in there like sardines. I have to go back and clean it up another day.
I just changed out my AUX battery. Great write-up! How did you get the battery and the bracket out? I could not fit my hand in there to get a good grip to pull it through. When I did have a hold on the battery and bracket it wouldn't come out. It didn't feel like it had much slack and I didn't want to pull on it to the point that I broke or disconnected anything that I wouldn't be able to reconnect. I was losing light fast so I just took the terminals off the old battery and attached them to the new battery and put everything back in the hole. To my surprise, there is a lot of room back there. Normally when you take these things apart all the wires and modules are crammed in there like sardines. I have to go back and clean it up another day.
I just used long nose needle nose pliers to twist it around and grab the bracket and pull it close enough to me until I could get a grip on it with my hand to pull it through. There is a good amount of space to work inside of there, thankfully. When I put it back in, I did the same thing until I could get it mounted firmly and screw it back in. You definitely need good light to see in there.
I just used long nose needle nose pliers to twist it around and grab the bracket and pull it close enough to me until I could get a grip on it with my hand to pull it through. There is a good amount of space to work inside of there, thankfully. When I put it back in, I did the same thing until I could get it mounted firmly and screw it back in. You definitely need good light to see in there.
Saved yourself a trip to the dealer and some $$!
Did you pop the battery out of the bracket with a flathead screw driver?
Did you pop the battery out of the bracket with a flathead screw driver?
Honestly, I don't remember but in looking at the battery (still sitting here on my desk), I must have since I see some scratches on the bottom corner that looks like I used the pliers to pop it out.
Mercedes auxiliary batteries occasionally operate in conjunction with the main systems battery or take over for it when that battery dies or has extremely low voltage. When the main battery has moderately low voltage, the vehicle computer connects the two batteries, allowing the main battery to return to normal, while charging the auxiliary battery at the same time. Many Mercedes vehicles feature a dual battery system to prevent failures in their combined hydraulic and electric braking systems.
Mercedes vehicles equipped with a dual battery system have several different modes, depending on the status of the main systems battery. The most common mode links the two batteries together, taking the load off the main systems battery and charging both batteries. The dual battery system also has two shutdown stages as well as an emergency mode that completely switches all functions from the main battery to the auxiliary battery.
During a stage one shutdown, some of the vehicle’s electronic components stop working. A stage one shutdown only disables a few of the vehicle’s electronic features, such as the rear windshield defroster and heated seats. A stage two shutdown disables internal power outlets and lowers the climate control fan speed. During an emergency mode shutdown, the battery control system switches all functions to the emergency battery, and only essential electronic systems continue operating at full capacity.
... because the W221/C216 electrical system was revised in 2009 and the configuration shown in the first post of this thread is applicable to 2009 and later models only.
And, the sole purpose of the auxiliary battery in these later models is to permit actuation of the solenoid that shifts transmission out of park when the main battery is dead. Otherwise, a car with a dead battery would be immovable.
what age car, I don't think you get this on early ones
or does it keep KLG alive ?
My car is a 2010.
For all the years I have owned the car there was a debate back and forth that the facelift cars didn't have an aux battery. They do, it was just moved to a different location. I got 11 years out of mine. My battery was shot. Certain functions didn't work like the hold feature. The AUX battery will also aid in starting the car if the main battery is low.
Last edited by The Thomas J; Jan 14, 2022 at 11:50 PM.
It is easy to get confused with the W221 battery systems. I encourage owners to read WIS function descriptions to figure this out without counting on contradictory comments in internet and discussion forums.
The pre-facelift W221 used two big batteries, a starter battery at the engine bay and a consumer battery at the trunk (see WIS gf54.10-p-0001-01sx). Later on (i think it aligned with face-lift), the starter and consumer battery were combined and a very small backup battery was added (as in the opening message) to feed power to the ISM (intelligent servo module that takes care of gear range selection, P/R/N/D, not the actual gear) in an emergency as lkchris explained (gf54.10-p-0990le). Then the 221 got the stop/start feature and the ISM backup battery was discontinued and a motor cycle size stop/start battery was introduced. This is on cars with the B03 option code.
the real issue is Mercedes using the wrong naming conventions
you can gave an interior toys consumer battery they call AUX
you can have an SBC emergency servo battery they call AUX
you can have a start stop battery they call AUX
you can have this battery they call AUX
all different sizes, in different locations, to do different things - they are just idiots
Yeah it took me some time to figure out that my "aux" battery is a big Group 49/H8. That's bigger than the starting battery up front!! It's like a 850 or 900CCA and the starting battery is a more modest 520CCA or so. Definitely a ton of battery capacity there. I may replace mine preemptively because I have noticed the dome lights shut off quickly if I didn't just drive the car. More research suggests that is because the aux battery voltage is below some threshold. No warnings on the cluster yet but given my road trip next month it might be best just to drop the $200 on it and avoid issues. According to records it may have been last replaced in 2012. These AGM batteries tend to just lose capacity over time and one day it falls below a threshold, but the truth is it's been well below spec for years. That was the case with my E55 main battery, which is similarly large (group 48 maybe?) but is the opposite of my S550. At first glance it seems the same. Bigger battery in back, smaller in front. But the architecture is completely different. So yeah people talk about these cars as if there is some standard layout. Not the case.
you don't get any messages on the 221. take it out, peel back the label, remove the VERY hard to remove screw top lids and put between 80 to 150ml of distilled water in each cell till the battery plates are covered by the white fibreglass tissue paper, recheck tomorrow (you want it puffed up and just damp looking - not taking a bath) and charge it WILL come back be better than a new one,
the New genuine merc ones you buy today dry out in 2.5 years and are worse than the one you took out by year 3
you don't get any messages on the 221. take it out, peel back the label, remove the VERY hard to remove screw top lids and put between 80 to 150ml of distilled water in each cell till the battery plates are covered by the white fibreglass tissue paper, recheck tomorrow (you want it puffed up and just damp looking - not taking a bath) and charge it WILL come back be better than a new one,
the New genuine merc ones you buy today dry out in 2.5 years and are worse than the one you took out by year 3
Damn that is what we used to do in Europe back then when I was growing up, didn't think it was still a thing 😲
AGM is almost exactly like a normal battery but the liquid is trapped in the fibre glass matt - it both stops the plates falling to bits and the liquid spilling - a nastier type is a gel battery
in that video clip you see the exact issue the black plates are visible and oxidising and the dry tissue isn't making much electrical connection to anything anymore
Thanks BOTUS for that top tip. Removed battery. This is in fact a Mercedes battery dated the 38th week of 2011 so my records were accurate from PO.
Fortunately the cell caps are exposed on these so I didn't have to mine for them. The most effective instrument to remove (they CLEARLY don't want anyone opening these after manufacture based on the one way cap ridges) was a snap ring plier, the 90 degree ones so you can mash it into the plastic with one hand and unscrew with the other. Worked pretty well actually.
Managed to remove back panel by bending it....will need to remove right side panel for reinstallation but I already did that. It was easy.
Oh, a Napa charger. Hope this is a multi-mode one and not just junk. Maybe this baked all the water out of the battery....
Says automatic but maybe I will remove, especially if I can find the plug for my Battery Tender.
Resting voltage after removal was around 12.1V. So....low but not dead or anything.
38th week of 2011 I assume. Almost 11 years old!
Put down with one hand, loosen with the other. You will struggle with needle nose or something blunt like that.
Seems dry to me....
Adding the finest Bavarian spring water.
I may have added too much but I'm not sure that will hurt anything
Date stamp on the terminal. That's cool.
On the charger. Curious how long it will take.
I'll report back with the results. I don't have a battery tester so my results will be how long I think it took to charge (if I pay attention), and then if the dome lights stop turning off prematurely. Best way to save $200 ever if it works!
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.
