it will take 6 months to come back to almost as new.... you shouldn't rush the refill, tomorrow it will take as much as it did today (I did mention that) it will take 48 hours at 4 amps a few times over the next few month and still eat more once its capacity starts to come back

and just nip those bungs - little fekkers don't like to come out, I put a bit of silicone lube on the two O rings

 

electronic boffins say the earth on the charger shouldn't be back to the battery... a different earth better allows the charger to identify the usual car drain and do a better job managing its charging up

Re boot trim if the retaining bit that wasn't on mine at the bottom, nearest the battery has been snagged off the trim.... then aside from the floor trim this rear most section can be pulled and put back with ALL other trim in place … JUST watch up for the nasty metal brackets that hold up the top boot trim (by the sub) often get snagged on refit and bend out of shape by bad garages

 

That would be correct if there was anything on the ground side. But apparently there isn't on the rear battery.

Goes straight to body ground.

 

Well now I've done it. I closed the trunk with the aux battery removed....won't open.

Nevermind.......

 

pop the sprung loaded bit on the key fob and out pops a key that goes in the lock on the left of boot lid by the chrome trim

 

Do you have any updates on this situation? I'm encountering the same situation right now.

 

A couple notes on this ...

Hold on to the battery bracket when removing the single hex headed screw ... its all that holds the bracket and it and (heavy) battery will fall when screw comes out unless you're holding it.

The silver item shown in the background in photo #4 is called central gateway control unit and it mounts to tabs on the battery holder. When I did mine, they were separated and that unit was just hanging under the dash. You may find that, too.

The screw mounting the bracket is really short, and I had trouble getting it started when remounting the bracket. There is plenty of clearance to install a longer screw. I did this, screwed it tight, and then used vice grips to hold it there while I removed it and reinstalled the original short screw. Optional, of course.

 

Hey WHPH28, thanks for the write up on how to replace Aux battery under dash in facelift w221. Excellent pics and instructions. I opted for the MB part, just to insure form fit and function, noticed it was made in Taiwan.

 

Awesome, glad you got it replaced and saved on labor from a shop doing it.

 

For completeness, months later I found battery acid all over the place in my trunk. It was a disaster and took forever to clean up with baking soda. I have no clue how long everything was soaking in acid. Zero new issues to report but I simply replaced the battery. YMMV but I would not recommend this at all.

 

on any AGM is supposed to be full of damp glass fibre tissue paper - its not meant to have a puddle on top

if you connect the drain pipe correctly any moisture should go on the floor outside

 

Right but I thought that a lot of water was going to be reabsorbed and was trying to shortcut the process, in anticipation of that. I will double down: not only did the acid thing happen (which, fine, was my fault), it didn't restore any capacity in the first place! Waste of time.

 

the original good quality battery or the junk peddle today ? any made in last 6 years are 1/3 or the quality they used 15 years back - we are in the tail end of civilisation.... its now a sick scramble for money, then the end is nigh... (for the majority)

 

Yes it was OEM Mercedes. 2011 date code or something.

 

Thanks ikchris for the model years when the giant battery in the trunk was designed out. I could inspect it and see if it is the original MB battery and that would determine if it needs replacing.
johnny

 

that's not correct - a real facelift after mid 2009 changed much of the car, twin batteries came and went much earlier, likely a mini update 2007-2008 when the aux RCAs turned up in the glove box

and it MIGHT be its more build spec - as sometimes some people seem to have twin batteries on much later cars than others....

 

It sounds easier to a look in the dash side panel for the smaller aux battery pack. The back panel removal behind the back seats looked like a pain.

johnny

 

on mine pulling that back panel is easier than people think - I think its supposed to be retained by one small bolt two foot in from the right trim panel along the bottom edge - when doing the battery clamp you have a spare bolt hole just further left that looks used... then last time I had the whole panel out I noticed a small rough bit on the trim panel (corresponding to the bolt location), I now figure it would have a bolt lug no one knows about so they snap it off when its first removed years before a second owner gets the car...

once that bolts is removed (snapped off) there is NOTHING else that holds the rear back trim panel in the car - just a crazy design that can't be pulled apart correctly (as the side panels should come out beforehand but they can't unless the rear panel is already out !!!) so you just use force on one bottom corner to pull it free squeezing past the side panels - the ONLY bit to remember on refitting - two cheap pressed metal trim brackets (part of the rear parcel shell) can get snagged on the top as you fight it back in - usually already mangled by the last bodging fool that ripped it out badly

the terminology used by merc for battery names is ridiculous

we should ALL refrain from using Aux Battery naming as it can mean different size batteries for different functions

e.g.

1) W211 Aux battery is a SBC brake booster back up battery
2) W221 Aux battery is the ISM back up battery (is what people here say this one under the dashboard is for - mine doesn't have one)
3) Later models Aux battery is the Stop Start logic control battery (which is located in the same place as the ISM battery on facelift W221s)
4) Consumer battery is an Aux battery if you used other manu sensible naming - for a non essential battery (aka not an engine management / starter battery)

 

What's funny is the three cars I have owned so far have all been one of the ones from that list. Specifically, I think, #1, 3 and 4.