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Have an 08 S-550 and have heard on here some time ago there might be more than 1 battery. Lately, when I first hit the starter, It almost hesitates before it fires up. I've regharged the battery under the hood, Is there another one??? I my 96 S-320 the battery was in the trunk. Thanks in advance.

Ron
 

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Mistertonyn:
Thanks for the timely reply. Do you know which side its on. It looks like a major job to remove the covering on the sides of the trunk. On my 96, there was an easy cover you could remove. In this one everything seems to be nailed down tight.

Ron
 

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Mistertonyn:
Thanks for the timely reply. Do you know which side its on. It looks like a major job to remove the covering on the sides of the trunk. On my 96, there was an easy cover you could remove. In this one everything seems to be nailed down tight.

Ron
You actually have 3 batteries, not many people realise this. The one in the engine compartment is for engine starting so have this one tested first. The one in the trunk is for all auxillary equipments and rarely fails, it is also a bitch to get to, here is an idea of how big a job it is;
 

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The 3rd battery is actually called the "additional Battery" and this is purely to keep the electronics and computers systems/programs alive and working if all power fails, here is a pic;
 

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Learn something new everyday....

Thanks Eric for the info.
How often does that 3rd battery gets replaced?
 

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The one in the trunk is actually not that hard to get to. Before you start unscrewing all those screws and removing trim pieces like I did, just pull the back board from the trunk and it exposes the battery. That board is free standing and with some effort just slides out.
 

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access to battery in trunk

The one in the trunk is actually not that hard to get to. Before you start unscrewing all those screws and removing trim pieces like I did, just pull the back board from the trunk and it exposes the battery. That board is free standing and with some effort just slides out.
You mean the backboard just pulls right out? No screws whatsoever holding it in place? I don't want to yank on it and have things breaking on me :big laugh:
The diagram eric provided looks complicated and he even said it is a bitch to get to that battery so who is right?
 

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You mean the backboard just pulls right out? No screws whatsoever holding it in place? I don't want to yank on it and have things breaking on me :big laugh:
The diagram eric provided looks complicated and he even said it is a bitch to get to that battery so who is right?
Correct, the panel at the rear comes off best if you pull from the top left first, no screws. You can easily access the rear battery but you have to work a bit more if you want to get it out.
 

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You mean the backboard just pulls right out? No screws whatsoever holding it in place? I don't want to yank on it and have things breaking on me :big laugh:
The diagram eric provided looks complicated and he even said it is a bitch to get to that battery so who is right?
Forget the diagram. Stick your left hand through the top left corner of the board, pull and your in. Less than a minute job. Just remember to pull hard.
 

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NO.

the battery under the hood is for starting only. the one in the trunk run all the other electronics in the car.

if one under hood is dead/or bad the key will turn on and everything look like normal, but wont start.
 

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I've had the starter battery tested, it checked out ok.
Still don't know what it is.
If the ignition lights come on but the engine will not crank then it is probably the EIS (Electronic Ignition Switch) that is faulty and it is very difficult to diagnose. Even for the dealer it is a headache.
 

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Forget the diagram. Stick your left hand through the top left corner of the board, pull and your in. Less than a minute job. Just remember to pull hard.
True; but (using the excellent info here while investigating my new-to-me 2010), I found it much easier to reinstall the panel by first removing the flat floor panel. Only two cargo net anchors hold it in.

For MY '07-'09**, there were three batteries as described in preceding posts - two large batteries (a 35 AH "Starter" battery, and a 95 AH "Electrical Consumer" battery) and a small 1.2AH auxiliary battery. Apparently beginning in '10 ('09 in Europe) the W221 went to a single large "Main" battery (95 AH, 850 CCA VRLA, or AGM - absorbent glass-mat) for starting and electronics, located in the engine compartment, and the small 1.2 AH Auxiliary battery. Those models do not have the "Electrical Consumer" battery in the trunk. The small 1.2AH Auxiliary battery is under the dash panel as shown at post 5 above (note that one removes the side dashboard fuse panel only to remove a bolt. It is then necessary to remove the under-dash trim to access the battery). See posts 12 and 18, http://www.benzworld.org/forums/w221-s-class/1938154-s550-4matic-dead-consumer-battery.html. For all models the small 1.2 AH auxiliary battery backs up the electronic transmission shifter and (possibly) other stored memory functions.

**Apparently some North American MY '09's were produced with only two batteries, as the '10 and later were. I do not know the date of changeover, but see posts 28 through 30 below.

Wampa007 has provided info on the specific sizes of the three batteries used in the North American MY '07-'09 HERE.

Models with ECO start/stop will have an "ECO start/stop" battery in the trunk (photo, post 5, http://www.benzworld.org/forums/w221-s-class/1938154-s550-4matic-dead-consumer-battery.html). As of MY 2011, ECO Start/Stop is standard on the S350 BlueTec models in some countries. See here (Note that seubill points out below that ECO Start/Stop was not made available for the W221 in North America).

In the photos of my car below, the upper left one shows the whole area behind the back seat. Below the white storage compartment is the rear SAM together with the fuse and relay module. To the left of it (second photo) is the Tele-Aid (MBRACE) module; the item in the rubber case is likely the pump for the drive dynamic seats. To the right of the SAM (last photo) is a pair of empty shelves.
 

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You actually have 3 batteries, not many people realise this. The one in the engine compartment is for engine starting so have this one tested first. The one in the trunk is for all auxillary equipments and rarely fails, it is also a bitch to get to, here is an idea of how big a job it is;
Like somebody said earlier, "Learn something new everyday". Excellent info. :beerchugr: :thumbsup:
 

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^ Eric's quoted post is true for '07-'09 models, including your '07 S65, and also all BlueTec models that have ECO start/stop; but gasoline models from '10 on do not have the battery in the trunk. See post 16 above.
 

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^ Eric's quoted post is true for '07-'08 models, including your '07 S65, and also all BlueTec models of any year that have ECO start/stop; but gasoline models from '08 on do not have the battery in the trunk. See post 16 above.
Correct, the aux battery was discontinued from 08. And for a good reason, the system was not functioning as well as expected, in fact it was causing too many problems. A good idea and well intended but it did not live up to expectations.
 

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Hi, Eric - just to avoid confusion (that I have helped to create by not paying enough attention to European vs North American model year designations) - for the North American models, there are three batteries through MY '09 for gasoline-powered versions. I checked the owner's manual I have for the '09 North American version to be sure, and it confirms the 3 batteries.

The single battery was used beginning in North American MY '10 (facelift). I have verified that with my own car. According to your post 12 at http://www.benzworld.org/forums/w221-s-class/1938154-s550-4matic-dead-consumer-battery-2.html the small backup battery was relocated to the engine compartment near the main battery. My dealership's chief tech insists that the '10 and all others have that small backup (auxiliary) battery that must be accessed as shown in your WIS at post 5 above - by removing the driver side dash panel fuse cover, then removing a bolt to free the panel under the dashboard, then removing the under-dash panel.
 
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