I bought a CTEK MXS 5.0 charger and used a cigarette lighter plug to charge my 03 SL55. It didn't work. What is the best way to trickle charge an SL55?

Directly from the consumer battery in the trunk.

Use the provided connector to mount directly to the battery and then run the wire to a convenient spot where you can attach the charger. I ran mine outside the trunk so I could conveniently plug in the CTEK.

So you place the charger on the rear battery? Does this charge both batteries at once? Why couldn't I charge both from the cigarette lighter?

Yes on the rear battery. The cigarette lighter is on a circuit managed by a controller that cuts it off when the engine is off. No charging the front battery will not charge the rear battery. I used a CTEK on my C5 for years with excellent results. However, regardless, I never ran a battery for more than 5 years before replacing it. I do the same with tires, irrespective of the miles. But I’m also a fanatic about 20k max on SL65 AMG ABC fluid change.

Are you saying that the two batteries are not connected with each other? Do I need 2 CTECKs...one on the front and one on the back?

Are you saying that the two batteries are not connected with each other? Do I need 2 CTECKs...one on the front and one on the back?

If you want to charge your starter battery, you will need another CTEK. However, the starter battery doesn’t draw down nearly as quickly as the trunk battery because it doesn’t have the parasitic draw of the trunk unit (it’s not connected to all the little convenience features). As a result, the starter battery doesn’t usually need trickle charging … unless, you’re going to let the car sit for more than a few weeks. Then, you would indeed be wise to connect the starter battery to its own CTEK

@Barry Hanna I have a CTEK connector plug hard-wired to my consumer battery terminals. I pop the trunk, connect the CTEK plug and just close the trunk. I have heard some people say that you can also charge the starter battery through the consumer battery, but I don't know for sure if that is correct. However, the car starts first time, even after many weeks of non use, so either it is true or the starter battery is very good at holding its breath for a long time. Can someone comment definitively?

@pauldoxey On some cars (eg Porsche) the cigarette lighter remains active with the ignition switched off, but on a Mercedes, it disconnects with the ignition. I keep my Porsche plugged into a CTEK, whenever not in use. Since the battery is in the frunk and it can only be opened via an electric switch, it is apparently a real PITA process to open it with a flat battery and to be avoided at all costs.

I use a battery tender pigtail hard wired to the rear battery. I just connect the pigtail close the trunk and plug it in. Has been working like this for 15 years. I only had to replace the battery once.

Folks: You cannot charge the starter battery through the consumer battery; they are not in-line.

The R230 (and R231) have two batteries because they have electrically actuated brakes. Two independent 12V sources are required for safety purposes. In my 2005 SL600 I had the "opportunity" to stop the car with no electrics. It was not pleasant nor easy, fortunately the car was moving at a very low speed.
When the car is off, the front starter battery is isolated with no loads connected. As long as the front battery is in good condition it will hold its charge for months. In 17 years of SL ownership I never had a need to charge the front battery. My front batteries have lasted seven to eight years then they are replaced. When the car is running the alternator will charge both batteries.
The rear battery when new will hold a charge (above 60%) up to two weeks depending on outside temperature. As you all know when the rear battery is discharged below about 50% the consumers go offline and the engine needs to be run up to 10 minutes to bring them back online.
The neat thing about the two battery SL's is they can always be entered and started with a completely dead consumers battery. The metal key opens the door and the key fob is placed in the ignition. Remove the pushbutton if present. Turn the key in the ignition and the IR transmitter in the key fob will start the engine with just the front battery. I have had to do this several times when a cell failed in the trunk AGM battery. It is annoying but it works. That is why I have never bothered with a trickle charger. If the consumers battery does not hold adequate charge for at least three days it needs to be replaced.