SL/R231: Second Battery?
Thread Starter
Senior Member
Joined: May 2012
Posts: 395
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From: Florida
AMG SLS & S coupe
Second Battery?
There is what looks like a second battery in the trunk. I've heard it's for the ECO start. However with the ECO engine starts, there is no sound of a starter. I understand that certain cylinders, those at, near or just past TDC, fire to get the engine start after an ECO stop.
Any input on this?
Any input on this?
Senior Member
Joined: Jan 2004
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From: Atlanta, GA
1988 560SL (sold); 2001 E430 (sold); 2004 SL500; 2009 CL550
Exactly, MB has been running a two battery system to support the extra load that the cars have for years. My 2004 has two batteries. It can also work to start the car should the primary battery be too weak to start the car on its own.
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From: S. Florida
'12 R197, '17 S63A4, '13 CLS4M, '99 A208 320CLK'99 CLK320A
The second battery is behind the passenger seat in the cockpit.
The "primary" (95AH) battery is the one in the trunk.
And yes, the secondary (small) battery behind the passenger seat is for the ECO start/stop system.
In previous SLs (e.g. R230) the primary battery was also in the trunk. The smaller (secondary) battery was in the engine compartment and was reserved for the starter and engine electrical needs only.
The "primary" (95AH) battery is the one in the trunk.
And yes, the secondary (small) battery behind the passenger seat is for the ECO start/stop system.
In previous SLs (e.g. R230) the primary battery was also in the trunk. The smaller (secondary) battery was in the engine compartment and was reserved for the starter and engine electrical needs only.
Thread Starter
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From: Florida
AMG SLS & S coupe
Both batteries in mine, are next to each other in my trunk below both partitions. Both batteries are mounted sideways with the main battery in the center. The second battery is to it's left, behind the driver.
Member
Joined: Oct 2007
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From: Bay Area, Ca.
2013 SL63
On mine the starter can be heard on an ECO startup. But just barely. I think the way they fire the appropriate cylinder makes it start up so fast that you can almost not even hear the starter. But it is definitely turning the motor over.
Senior Member
Joined: May 2010
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From: S. Florida
'12 R197, '17 S63A4, '13 CLS4M, '99 A208 320CLK'99 CLK320A
In any case, it is not the second battery.
The second battery is exactly where I said it is.
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Thread Starter
Senior Member
Joined: May 2012
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From: Florida
AMG SLS & S coupe
Is the second battery in the passenger side storage compartment, behind the seats, that doesn't open?
Thread Starter
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Joined: May 2012
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From: Florida
AMG SLS & S coupe
Senior Member
Joined: May 2010
Posts: 357
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From: S. Florida
'12 R197, '17 S63A4, '13 CLS4M, '99 A208 320CLK'99 CLK320A
I'm pleased that you now know that the battery is where I told you it is.
Hope my posts helped you.
Thread Starter
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From: Florida
AMG SLS & S coupe
No need to react negatively. I just wanted to know what the box was next to the battery. I asked my service advisor at M-B the last time I was there and even he didn't know.
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From: pittsburgh
'25 740i; '15 targa4s; '24 cayenne
huge learning curve on a new model like the 2013 sl550. for everyone alike: owners, salesman, mechanics, etc. part of the (a-hem) "charm" of owning a first run batch of a new chassis model. we'll all get there.
Senior Member
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From: S. Florida
'12 R197, '17 S63A4, '13 CLS4M, '99 A208 320CLK'99 CLK320A
There is a tendency, especially with salesman, to bluff (guess) at the answer.
At a Palm Beach County MB dealership, for example, a so called top salesman asked me what vehicle I was talking about when I asked a question about my SL and I had referred to it as a R231.
I put a very high value on those who communicate factual information secured only from a proven reliable (usually written) source. Some questions are hard (e.g. how does the SL ECO stop/start really work?).
Some questions are really easy (e.g. where is the 2nd battery?)
The later is not a learning curve question, it's in the owner's manual.
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Joined: Dec 2009
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From: USA
10 speed bicycle
Normally these two batteries are completely isolated, only for charging or undervoltage are they linked in parallel by battery cutoff relay K57.
Once again see previous quote, under normal circumstances the starter battery powers ONLY the starter armature.
Thread Starter
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From: Florida
AMG SLS & S coupe
Why don't I hear the starter motor when the ECO restarts the engine? All I hear is chug-chug and the engine starts.
Last edited by Daytona-flyer; Feb 18, 2013 at 06:17 AM.
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From: USA
10 speed bicycle
This is the reason you don't hear the starter as it is never really making a full revolution in the ECO start/stop process.
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My 2013 Sl63 has a hidden, very small, second battery behind the passsenger seat. ITs sole purpose is to provide electrical power to the transmission and allow the selector to be moved into or out of park should the main battery fail. Other then that there is really only one battery for everything else.
Super Member
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From: USA
10 speed bicycle
My 2013 Sl63 has a hidden, very small, second battery behind the passsenger seat. ITs sole purpose is to provide electrical power to the transmission and allow the selector to be moved into or out of park should the main battery fail. Other then that there is really only one battery for everything else.
Partially correct, that second battery is known as G1/13. I did forget to mention that this battery also will provide power to the Inteligent Servo Module (ISM A80) in the event of On-board electrical systems failure. This however will only engage park and is done so automatically once the key is removed from the ignition switch (EZS N73). For vehicles with KG park is engage once the driver door is open. It is known as a fail safe and will not allow you to disengage park.
The transmission still receives it's power from the main battery G1.
Please reference the following WIS documents:
GF07.10-P-1041MMF - Engine Control Start/Stop function
GF54.10-P-1040RK - ECO Start/Stop energy Management
GF27.10-P-3029AHJ - ISM Emergency P function
Web ETM:
PE15.00-P-2103RKA
*Note: This is only for vehicles with SA code B03 ECO Start/Stop. Vehicles without ECO (ie: other countries) will still have a seperate battery G1/7 which will serve the same function for the ISM.
Last edited by MBtech1098; Feb 24, 2013 at 10:40 AM.
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W212 FL
Thanks for everyone for your informative replies 
It never dawned on me that 2nd battery would provide enough power for such things just in case , what an engineering marvel
Thanks , MBTech for your replies as well
It never dawned on me that 2nd battery would provide enough power for such things just in case , what an engineering marvel
Thanks , MBTech for your replies as well
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Hi California J. With just 600 miles on it I have been going by the book for the break in period. I traded my 2004 SL600 and this car feels and is much more luxurious. The steering feel is good especially when you select S or S+. I was concerned about the new electric steering feature. The noise is great, just like the reviews say it sounds like a Nascar Race Car although I have only been up to about 4500RPM and I expect it will get better. I have to put a few more miles on it before I can give a real report as I am still in the "new"stage. Silver car with red and black interior.
