Hello everyone,
I am winterising my 2013 SL 550 R231 for about 5 months and I need some in formation as I have read conflicting answers regarding my question.
My question is trickle charging the car's battery or batteries.
I know that there is the main battery in the trunk, it can be reached (charged) through the charging posts in the engine compartment.
The second battery is located below a compartment behind the passenger seat.
Witch battery needs the trickle charge or do both need be charged?
Is a 1 amp. trickle charger (s) powerful enough ?
Thanks for your help

SLDOUG,

I don't have a definite answer but a pretty educated one for you. I own a 2003 SL500 and it too has two batteries. The accessory battery in the trunk and the startup battery under the hood. I leave the 2003 in Florida and only use it in the winter months. I use a trickle charger on the accessory battery in the trunk and every time I go to Florida it starts fine and runs fine. I think in large part because I keep the trickle charger on the accessory battery in the trunk.

I now own a 2013 SL550 and think I will do the same thing when I go to Florida for the winter and that is put a trickle charger on the accessory battery which is located behind the passenger seat.

I look forward to any more experienced R231 owners who have needed to leave their cars garaged for months at a time, especially during the winter.

Thanks!

How do you (easily) access the accessory battery behind the passenger seat in order to attach charger cables?

Thank you for your help,
Best Regards
SLDoug

Slightly different answer from me. Doesn’t matter how many batteries the car has.
You usually use the charge points in the car if it has them. That means under the hood. The cars charging system takes care of batteries.
Just like in dealer showrooms (normal times when dealers have inventory) cars are usually plugged in on the charge points.

Some relevant info in this thread starting at post 23. https://mbworld.org/forums/sl-class-...tery-type.html

Great info! But it makes the point via measurement that it’s not necessary to charge the accessory battery anyway. We have had our SL’s sitting for up to 5 months for many years either with a single trickle charger to the charge point or nothing at all (just starting the car occasionally and letting it run for a bit but that was more for the R230’s).
Eventually we decided that keeping the SL in storage for the winter was a waste and put on some snows instead. They drive great in the snow
Something we reversed for the AMG GTC. That car is back in storage and gets trickle charged via the cigarette lighter in the trunk…

Agreed.
The voltage drop on the Aux battery (measured at approx. 1.5 mV a week in warm weather) is so low that it hardly seems necessary to keep a maintainer on it full time while in storage.

It’s been suggested that the Aux battery will take a charge from the battery posts under the hood, but only once the Aux battery falls below a threshold of approx. 11 volts. 11 volts is too low to allow an AGM battery to sit, so topping up sooner (at 12V) from the Aux battery terminals behind the passenger seat is probably a good idea if and when necessary.

That’s the confirmation I was looking for. There are many contradictions regarding this issue for the 2013 SL550;R 231
I was under the impression that you connect the trickle charger directly to the charging posts in the engine compartment ( passenger side firewall )
and this directly charges the auxiliary battery in the trunk without direct connection to it needed. No need to connect a trickle charger to the main battery behind the passenger seat as it will draw power from the auxiliary battery in the trunk if needed
i hope that finally I have it wright, let me know Wolfman
Best Regards
SLdoug

Just to clarify… the configuration in my above post is for a 2019 SL 450 (R231).

1. The Main (large) battery is in the trunk and the Aux. (small) battery is behind the passenger seat.
2. The battery posts under the hood are connected to the Main battery.
3. The Aux “may” be charged from the battery posts under the hood but only if the Aux. drops below approx. 11 volts.

Note: No. 3 is believed to be true; however, as of yet there is no objective data to confirm it.

Number 3 is correct. The aux battery through a one way connection can be charged by the main starter battery. Just connect a battery maintainer to the posts under the hood and all will be fine. As mentioned above cars sit from the factory to the dealership for long periods of time and it would be crazy/expensive for the dealerships to dive into the compartment behind the passenger seat just to charge the aux battery.

THIS makes much more sense. Thanks for the confirmation of what would seem obvious!

I found a CTEK pigtail had been installed on my SL by the previous owner so I've been connecting a CTEK to it whenever the car is parked for more than a week. I have noted that it takes only a few minutes for the CTEK to show that it is fully charged unlike the main battery which usually takes several hours. I think I'll try using only one charger connected to the charging posts under the hood this winter. I will trust hornethandler.

What is the CTEK pigtail attached to?

The eyelets permanently connect to the battery posts and the quick connect/disconnect plug connects to the charger... see link below.
antigravitybatteries.com/products/accessories/battery/chargers/ctek-comfort-connect-harness/

how long time can the sl500 sit in garage in minus ,before batteri drop? I read somewhere the 2013 models have aux batteri inside fotwell passenger side, Then later models moved pos bhind passenger side.

Depends on the condition of your batteries but I would definitely use a battery maintainer on the terminals under the hood. Supposedly it will also keep the auxiliary battery behind the passenger seat from going below a level which would render it ineffective.