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.

 

i use a battery tender on my auxiliary battery too.

 

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.

 

No, circuits are separated 100% when car off, only during operation when the starter battery is too low a relay closes and connects the rear to the front to start the car...
The rear gets charged from the alternator, and the front gets charged via BCM module DC/DC charger...
Only rear needs to be on trickle charger, I top up the front starter battery only once over the 5/6 month winter period.

 

If you only have one tender use it on the auxiliary battery. If you have two then do both. Don’t disconnect the batteries!

 

I use a CTEK on each battery simultaneously. '04 SL55 AMG

 

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.

 

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.

 

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:

http://benzbits.com/R230/BatterySystem2009.pdf

 

Good read. So the batteries are separated when not running. Hence, two tenders are needed. Thanks Rodney!

 

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.

 

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.

 

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.

 

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

 

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.

 

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.

 

Yes, of course. The rear battery is drawn from constantly while the car is parked. The smaller front battery is not, not at all.

 

This is not true!! A simple tester indicates (atleast on my 2006 SL65) that there is a constant 25mA draw on the front battery, even when just sitting.

 

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.

 

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.

 

The front starter battery on my 2006 SL65 indicates a constant 25mA draw (tester confirmed) even when just sitting. Something is on if the terminals are hooked up? And, if one is storing their car for months, my father always taught me it is so much safer just to unhook a battery terminal and then put on the battery maintainer.

 

Are you sure the 25mA you see is not actually a current going IN to your starter battery from the BCU?
According to the electrical schematic, the only things connected to the front battery +V is the starter solenoid, K57 which should be OFF, K75 which should also be OFF, and the output of the DC/DC converter which can only feed current into the starter battery to keep it charged up!

Of course someone may have added a circuit to the starter battery or the BCU is faulty!

 

There's an exception to the single mindedness of the starter battery: it backs up SBC power while the Hold function is active.

 

I would get a battery maintainer. "Unhooking" the battery was a great idea on a '48 Cadillac.

Not a great idea on this car. Battery disconnect requires resets of steering angle, windows, and clock. See the applicable page in the manual (405 in mine).

 

The BCM will switch the starter (front) battery into the circuit when the consumer (rear) battery is drain or disconnected. Doing an amp test on the starter battery with the consumer battery disconnected is not a valid test.

 

Only did the test with the rear battery disconnected to show the amp draw/charge was not coming from the rear battery. Since both tests showed the same exact amp reading, it cannot be a charge coming from the rear battery. And with nothing to charge only the front battery, I'll go back to my original assumption that it is indeed a draw, not a charge.

 

You are right - it's a draw and not a charge. But its a draw on the front battery only when the rear battery is dead/disconnected. When the rear battery is online and supplying more than 11v, the front battery is totally disconnected from everything.

 

I cannot agree, as with the rear battery (1 year old) hooked up and on the maintainer, I am still seeing the identical 25mA draw from the front battery. No idea where it is coming from??

 

This may just be the base draw that the BCM place on the battery to keep check on its charge level. 25mA is not very much - 1/3 of a watt. A small indicator LED draws more than that.

 

I took both batteries off my car for 6 weeks when doing a front suspension refresh…

Expected the worst and had my scanner ready for all the DTCs when I reconnected the batteries…

I must admit I’ve fitted a battery isolator to prevent any BCM issues when trickle charging the rear…

The front battery is tough as old boots and starts the car even when left in the garage for 6 months

 

found this thread trying to verify OK using CTEK on both batteries at the same time.

interesting several people find a 25mA draw on the starter battery. I have that too. had read there should not be any draw on the starter battery. but after several days of sitting would find it too low to crank.

totally disconnected from the BCM to confirm not coming from the rear.

and had a situation being gone for a while to where the consumer battery was totally drained along with the starter battery. AGM batteries dont like that. resurrected with a dumb charger but then kept getting consumer battery error symbol. new battery now.

so joining the dual CTEK club.