I wonder if it's possible to charge the rear battery thru the rear 12v outlet, I know it doesn't make sense, however I've seen pictures that people plug their trickle charger to the 12v cigarettes outlet and I wonder if that is possible? Because I really like the idea of not open the trunk mat.....

 

Editing my post; I thought I was reading that you wanted to charge the rear trunk battery through the cigarette lighter from the car's own electrical system. A loop.

Re-reading, I did what Gene said below, I just put the hard-wire plugs onto the battery posts.

 

I jus hard wired the pig tails for the Battery Tender to the rear battery.
I keep it plugged in anytime I'm not going to be using the SL.

 

Yes you can use the 12v outlet in the trunk to charge the battery.

Mercedes actually sold the chargers with the lighter fitting for that purpose.

It must be only a trickle, maintenance charger. This will keep the rear battery topped up, then the rear battery will keep the front starter battery charged via the onboard DC/DC converter.

 

So I have been using two battery tenders. One on the back and one on the front. No problems what so ever. You are saying for sure without a doubt all I need is one on the back battery and the starter battery will be kept charged too?

 

Yes 100% sure unless you have a problem with the DC/DC converter which is very unlikely.

 

Man thanks for the incite. I would much rather just have one charger on.
Im going to do a little experiment. I disconnected the front charger. As of right now the front battery is at 13.02 volts. I will monitor it to see if it drops.

 

It will drop a little until the DC/DC charger comes in. I am not sure exactly what the voltage trigger point is.

Try to monitor it over a few days.

 

What the heck, seems like you can keep the rear battery maintained by using the charger in the trunk to plug into the car. I went through all the trouble to do this: https://mbworld.org/forums/sl-class-...r-battery.html

 

TTtodd let me know your findings, I would like to know how far it drops before the DC/DC converter kicks in.

 

It has only been a couple of days. I will continue to monitor every day.
So far I started out at 13.02 volts on 2/22. Yesterday it dropped to 12.76.
Today it is at 12.77.

I think it will take a few weeks of data to really know whats going on.

 

It seems like it has already started to float the voltage as it would not have increased any other way.

 

NOTE 1 : discharging a battery to a point too low will damage the battery.

NOTE 2 : SL55 AMG dual battery systems are separate (according to the wiring diagrams). You require to charge both separately (I checked that the starter voltage does not rise while charging the system/consumers battery).

NOTE 3 : when “jump-starting” only use front battery (never use rear one).

There are two batteries in the SL55 (front - starter battery only; rear - system/consumers battery used for everything else).

The starter battery is dedicated to starting the car, so depletes quite slowly and seldom requires recharging. However, the systems/accessory/consumers battery requires more frequent charging, as it continues to draw down while standing, depleting rather quickly (about 0.14 volts per day).

It appears that Mercedes-Benz split and separated the batteries, as so many people store their cars for long periods, which would have rapidly depleted the starting battery (lots of electronics in the car). These two batteries are connected through a battery control module, which regulates the charging and discharging of the two batteries, as well as handling load management. The battery control module measures the load capacity of the main battery, actual current flow to loads, and can take active measures to stabilise the electrical system. These measures include increasing idle speed, connecting the auxiliary battery to the system via a relay. or shutting off nonessential consumers through the CAN Bus. This means that, under certain electrical conditions, some electrical features may not work, while at other times they function normally".

If the car is used daily, then there are no problems, but if car left sitting for a few days or more, the system/consumers battery will be depleted with everything “off-line”, until it is driven a few miles, to recharge system. Note, main starter battery (up front) holds its charge well (as it’s only for engine starting, and quickly replaces any lost charge, as soon as it’s running).

 

Geo this is not correct.

The r230 has a DC/DC converter between the two batteries from rear to front to help the starter battery if it gets low. The rear battery will even go it to limited function mode if the starter battery gets low in order for the car to be able to start so you don't get stranded.

 

From my experience (before replacing my front flat battery) :

1) Rear battery was already new and fully charged (always kept on a trickle charger) but front battery continued to drain and would not take any charge from fully charged rear battery or from its trickle charger.

2) The only way I could get charge into the front battery, was by directly connecting to it, a separate front trickle charger.

3) Now, after replacing the front battery, I don't even need a trickle charger on it, but still have to keep one on rear battery.

My understanding is that the two batteries are not interconnected, but maybe I am wrong.

 

You just proved exactly what I am telling you, which by the way is correct. The starter battery goes bad it might take charge a little better from a direct charge however it is on its way out. It is a cheap small battery.

The reason why since then you have not had to charge the front battery is because it is being kept topped up from the rear battery.

 

F1BHP - You are totally wrong.

The DC/DC converter connects to both batteries, but cannot send any charge from rear to front.

Just to confirm, I checked my new front battery (front trickle charger disconnected) and with a trickle charger on rear only.

Result - rear battery remains charged, while front receives no top up.

Follow this link for more on subject :

http://www.benzworld.org/forums/r230...e-battery.html

I'm tired of discussing this further........Geo.