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I have a 2009 MB SL63 w/two batteries - the starter battery up front & auxiliary battery in trunk. I will be leaving the car at another location (away from my permanent home) for several months and want to install a battery tender(s) while I am away. Do I hook it up to the auxiliary battery in trunk or the starter battery in front or both? Also, if I disconnect the battery during the install do I lose all of my electronic settings?
 

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2008 SL550R
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130 Posts
I use a battery tender on my auxiliary battery (trunk mounted) ‘08 sl550, I don’t disconnect anything and I don’t have any problems to report. 5th winter CTEK charger.
 

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2009 SL65
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91 Posts
Yes. CTEK trickle charger maintains the rear battery nicely which I believe also charges the starting battery in front too for my 2009 SL65. Been doing that for 16 months now even though I've only put 350 miles on my weekend warrior in that time.
 

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2005 SL65 plus 13 other non MB toys
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763 Posts
If you only have one tender use it on the auxiliary battery. If you have two then do both. Don’t disconnect the batteries!
 

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2004 SL55 AMG DEsigno Edition, 2009 ML350 4matic
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I use a CTEK on each battery simultaneously. '04 SL55 AMG
 

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2007 SL550 (R230) and 2012 SClass Designo (W221)
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If my understanding is correct, there is a BCM (battery control module) that regulates the charge flow so that the front starter battery is charged if needed when rear battery is being charged by electric charger, thereby eliminating need for 2 chargers, just the rear is needed to maintain both batteries. Perhaps one of the board gurus can confirm.
 

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2009 SL550
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138 Posts
If my understanding is correct, there is a BCM (battery control module) that regulates the charge flow so that the front starter battery is charged if needed when rear battery is being charged by electric charger, thereby eliminating need for 2 chargers, just the rear is needed to maintain both batteries. Perhaps one of the board gurus can confirm.
Is MNSL550's "understanding" correct? I have heard the above from others and hope he's correct because I keep two Battery Minders connected to my SL in winter when the car sees very light use (one on the starter battery and one on the 'comfort' battery).
If it's redundant to keep a Battery Minder connected to the starter battery because the BCM is watching over it I could have my 'extra' Battery Minder keep my lawn tractor ready for duty once Spring arrives.
 

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2005 SL65 plus 13 other non MB toys
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I have never heard that before. I keep two on mine. Would also like to know if this can be confirmed by a reliable source.
 

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Coupe/Convertible Forums Moderator
CURRENT: 2011 SL550, 2011 C300 FORMER: ML350, CLK550 Cabriolet, C240, ML320, 300TD
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The BCM isolates the starter battery except in two situations. One, when the engine is running and the alternator is putting out, it can bridge the starter battery to keep it charged. Two, if the consumer battery is very low, it will bridge the starter battery to supply additional power. This document below explains it. It's for MY2009+ facelift cars, but the differences are minimal for earlier cars:

 

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2005 SL65 plus 13 other non MB toys
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Good read. So the batteries are separated when not running. Hence, two tenders are needed. Thanks Rodney!
 

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2003 SL500, 2003 SL55, and four R129s
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The starter battery sources no current at all with the engine off. Perhaps if the car is going to sit for longer than 6 months a maintainer would be helpful; otherwise, I think it is not necessary.
 

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2009 SL65
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91 Posts
Thx.
Is this the same for all years of the R230, or is it different for the 2009+ facelift years?
I keep reading various posts and I assume the answers which are given pertain to the original poster and his/her car model and year. Sometimes I wish there was a separate R230v2 section to deal with any of the differences.
 

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2003 SL500, 2003 SL55, and four R129s
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Is this the same for all years of the R230, or is it different for the 2009+ facelift years?
It's the same for all years.

The functional description supplied by Mercedes for the two-battery system is different for early and later cars. According to M-B, on the early car the power supply module switches some consumers off when the rear battery voltage falls below 10.8 volts DC. Recently my consumers were offline and I measured a voltage of 11.5. That voltage threshold is the only thing I have found in the functional description which seems to be incorrect.

As an aside, I find this feature annoying, and I question whether it is necessary -- certainly for myself, at least. Using Star Diagnosis I have looked for a setting to change the threshold voltage and found none.

Just as a smartTOP module fakes the roof controller into thinking the vehicle is stationary when it is in fact moving, I'd like to have something for the vehicle battery control module which would allow me to select the threshold for consumers switching off.
 

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2003 SL500
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222 Posts
One Ctek Polar hooked to Aux Battery during the winter months in the North East, been doing it for years, no issues.
2003 SL500
 

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CURRENT: 2011 SL550, 2011 C300 FORMER: ML350, CLK550 Cabriolet, C240, ML320, 300TD
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One thing they changed on the facelift was to not display the "consumers offline" message on the MFD during low voltage conditions. I think that was a bad decisions. I'd rather see the message rather than have to guess what is wrong when, say, the steering column adjustment won't work.
 

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96' S500, 09' S550, 06' SL65
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206 Posts
I don't believe charging the consumer (trunk) has any effect on the starter batt. I keep the consumer tendered continually at home and the front (starter) charged every now and then. Seems like the bus system causes static drain on the consumer to the detriment of anyone without a tender.
 

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2005 R230 SL350 (M112)
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27 Posts
Yes, of course. The rear battery is drawn from constantly while the car is parked. The smaller front battery is not, not at all.
 

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'04 SL500; '05 E500; 05 E320 Wagon 4-matic
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In N.E. my R230 stays in storage for the winter, and has for many years now. Long ago I used two CTEK chargers (4.3 & 0.8), but didn't install the 0.8 on the starter battery one year when I needed it for the generator and it didn't matter in the spring, so I no longer bother (it's on the generator permanently now). The rear battery requires a charger of some kind if the car is left anywhere unattended for longer than a week. I am a serious fan of the CTEK line of trickle chargers. Great stuff.
As the front battery dies though, the car acts rather odd. I'm guessing it dies of age and heat exposure, like any battery mounted up front. Although it started and ran, I was having all sorts of peculiar problems (random brake and other warnings) until it just wouldn't (re)start one day at a store. Jumped it to get home, replaced it (Advance Auto - in stock) and all the odd problems went away.
 

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2003 SL500 USA model, Auto Transmission, Stock, Aegean Blue Metallic
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1,409 Posts
Stanley Re-chargeit 15 amp is a great solution... no affiliation. It does trickle charge, a cool battery reconditioner, 15 amp rapid charge, 40 amp engine start, alternator test. It automatically selects charge rate and adjusts from rapid charge to trickle charge. Walmart online had a really low price.
 
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