Correct, mine (MXS7) has a ‘supply’ mode which the 5 lacks.

 

WOW! I didn't even know that this car had two batteries! I've had many cars but never one with two batteries. I could understand it if it was a Ferrari or some exotic super car but This car?
I use to do all my own work on my cars but getting old and in the way I just can't do it any more. I'm just disappointed that this is another thing to throw money at when it goes bad. It is what it is. You just have to live with it. The price you have to pay for a really nice car these days.

 

Auxillary will charge from the jumper posts under the hood.

 

Are you sure? I was told the auxiliary is only charged when the car is being driven.

 

100% certain on mine. Might be different on earlier models. Had it 4 years and it spends a lot of time in my garage & I have never charged the Aux separately.

 

Hi all,

I am reviving an old thread here and am looking for some help.

My 2017 AMG SL63 will no longer hold a charge. I have some questions about what to do, and I will provide some context.

Context:
(1) June 2016 Production
(2) 03/2019: Mercedes Dealer replaced starter battery under warranty. They replaced it on their own while car was in for a different issue - repair order said the engine was “hot starting,” and the battery failed testing and exhibited low voltage. Part number was 000541100; apparently it has been superseded by part # 0019828208.
(3) I am a car collector with more than 10 vehicles, and I maintain the batteries in all my cars using Deltran Battery Tender Plus (now with the new generation units without the internal circuitry) and the OBD II socket adapter. From McLaren to Porsche to BMW to Audi to Aston Martin to Mercedes — and the list goes on — the OBDII socket adapter works, so I do not wish to debate this as a possible variable. It’s a very convenient way to maintain batteries — a few years ago when I discovered this device, I was happy that I would never again waste time and money installing pigtails, which I mostly did myself but sometimes paid others to do. I recommend this little device to everyone. The OBDII port is always live, so don’t keep something plugged into it with the engine off unless it’s supplying power - even an adapter device plugged in there without any draw will cause a small draw on the battery.
(4) On this SL63 as well as on all the other cars, the OBDII socket adapter interfaced with the battery tender completely charges and floats the battery, as the battery tender displays “charge complete” or float cycle once the battery reaches full charge level.
(5) A few weeks ago, at 6:30am on the morning of a road trip in the SL63, for the first time ever the car wouldn’t start. Go figure. I jump started the car using the remote terminals in the engine compartment (these are put there by Mercedes as extensions from whatever battery), and I had a successful road trip without issues. I did not notice at that time whether the battery tender had been doing its job; but I can say I out the top up and down a few times with the engine off, which is a no-no since they sequence uses significant power. I wasn’t thinking.
(6) Over the past few nights, I’ve noticed the battery tender never reaches full charge; it becomes warm; and it even displays an error sequence (flashes from green to red) — this means the thing isn’t charging.
(7) I have never seen a battery warning light.
(8) I understand that Mercedes-Benz calls the main battery the “starter battery,” and the little battery the “auxiliary battery”

Questions:
(1) After reading this thread, my understanding is that the correct description of the two batteries is as follows: the smaller battery behind the passenger seat is responsible for keeping the vehicle alive while the engine shuts off during Eco start/stop, while the main battery in the trunk is responsible for starting the engine. Do we agree?
(2) It was previously mentioned in post #78 on this thread that the remote positive and negative terminals in the engine compartment interface with the auxiliary battery. I think that statement might be an error. Please confirm whether post #78 was incorrect. It seems to me that if I used the terminals in the engine compartment to jump start the car, then they would be interfacing with the main battery, since the main battery is responsible for starting the engine.
(3) Do you think my problem is with my main battery? After reading this thread, I tend to think it is.
(4) Suppose I replace the main battery. Would putting a trickle charger (1.25 amps) into the OBDII socket be sufficient to maintain minimum voltage for electronics memory etc, or should I connect 6-8 amps to the remote terminals in the engine compartment?
(5) Is there any required procedure or sequence to “register” a new battery? I know that on BMWs, for example, a computer is needed to tell the car the battery was replaced (battery registration).
(6) On my repair order from 2019, it says “reset charge balance on front sam.” I don’t know what that means.
(7) Do I need to concern myself with the auxiliary battery at all? I tend to think it’s fine for now.
(8) Under normal conditions, does trickle charging the main battery also result in a charge to the auxiliary battery, or is the auxiliary battery charged only by the alternator?

 

1) Yes
2) Possibly. No wiring diagrams.
3) If the battery is relatively new, I imagine the problem is elsewhere. It is easy to get to and you can put a volt meter across it and see what voltage it is (lwait for several hours (overnight) after driving or charging). You can even check the voltage at the jump start points. Should be at least 12.6V.
4) I’d use the jump start points.
5) Not needed. I replaced mine - no issues.
6) ?
7) I believe you will get a warning if there is an issue. Mine is now 5 years old, still holding 12.8V. It is a pain to get to and check.
8) I thought so but am not 100% sure anymore.

Your problem might be charging through the OBD port (the technology changes so quickly that the OBD in your car might not be suitable for charging).

 

A few comments:
1. Don't try to charge a discharged battery with a tender. Use a charger. If it won't charge it needs to be replaced.
2. There is still no agreement on how to maintain both batteries. Some use the jump starting terminals under the hood and others, like me, put a pigtail on the main battery and use it.
3. Replacing the auxiliary battery is a bit of a pain but once the cage is removed it is very easy.

If your main battery can be fully charged but it loses its charge quickly it could be a bad battery but could also be caused by a parasitic draw from the car. This can be difficult to track down.

Hopefully others can be more helpful. Good luck.

 

(1) Yes. For stop/start the aux battery is momentarily switched in to take over accessory loads while the main battery re-starts the engine. The aux battery also directly powers the ignition "switch" module and the transmission gear selector module. This would allow you to shift into neutral in case your main battery is dead and you need a tow.
(2) Not true. The under-hood terminals connect directly to the main battery in the trunk. That's the battery that runs the starter..
(3) Possibly. You didn't mention the symptoms when the car wouldn't start. Was there no response at all? Did you hear clicking sounds? Did the dash display light up indicating ignition was on? You may have a defective main battery. But, as I mention above, the aux battery powers the ignition switch module. If the aux battery is flat it might prevent you from operating the ignition. I suggest checking voltages on both batteries after the car has sat for at least a day. It's possible you need an aux battery.
(4) With everything shut down (and key fob out of the area) I would expect the OBDII approach should work. I did this by placing a UPS battery on the terminals under the hood while I changed the main battery.
(5) There's nothing required for Mercedes that I know of. The system will eventually figure out you installed a new battery.
(6) At the shop they may have a function in the STAR system that speeds up the process of recognizing a new battery. I believe the purpose of this is so the SAM can detect when the battery is failing Overall, the SLs seem to be especially bad at detecting and reporting a bad battery. Sometimes it does, but many times it doesn't and other symptoms appear. On my 2013 SL I got a message that the main battery was defective and replaced it. No issues. If you scan the forums though you'll see many cases where it wasn't that clear cut. Maybe there is something to be said for having the shop reset the SAM?
(7) Yes, you should at least get into the aux battery compartment and check the voltage. There's a good chance it's bad because....
(8) The aux battery will not get charged by a trickle charger on the main battery. So your aux battery is getting charged only when you're driving the car. There's a SAM-controlled switch that connects the aux battery to the main battery if it detects that the aux battery voltage is low. This happens only if the engine is running. Many people who store their SLs for long periods put pigtails on the aux battery so they can attach a second trickle charger.

Most people immediately shut off the ECO function when they start the car. But, you can potentially detect a bad aux battery by leaving the ECO function on for a drive. If the aux battery is weak the system should tell you that ECO is disabled or that accessory functions are "limited". This may not happen until the engine is warmed up and you come to a stop. That tells you the aux battery needs replacing.

Concerning using the OBDII port for charging. I think that's fine as long as the charge current is limited, so trickle charge only.

 

I know from keeping my car on trickle charge over the last 5 years (only 18,000 miles in 5 years) that the the Eco Stop/Start is always immediately available when I drive off. The opposite is true is I do not have the car on the trickle charger. My aux is still reading abut 12.8 V even after standing for several days. I replaced the cranking battery a couple of months ago.


Info from the owners manual regarding Eco conditions:

I’m still not convinced that the AUX is not also getting topped up.

• If all conditions for the automatic engine switch-off more are fulfilled, the ECO symbol is shown in the multifunction display.
  If all conditions for the automatic engine switch-off more are not fulfilled, the ECO symbol is shown in the multifunction display. If this is the case, the ECO start/stop function is not available.

If indicator lamp is off, the ECO start/stop function has been switched off manually or as the result of a malfunction. The engine will then not be switched off automatically when the vehicle stops.

Automatic engine stop

If the vehicle is braked to a standstill with the transmission in or , the ECO start/stop function switches off the engine automatically.
The ECO start/stop function is operational and the ECO symbol is shown on the multifunction display if:

• the indicator lamp in the ECO button is lit green
• the outside temperature is within the range that is suitable for the system
• the engine is at normal operating temperature
• the set temperature for the vehicle interior has been reached
• the battery is sufficiently charged
• the system does not detect moisture on the windscreen when the air-conditioning system is switched on
• the bonnet is closed
• the driver's door is closed and the driver's seat belt is fastened

If all conditions for automatic engine stop are not fulfilled, the ECO symbol is shown.
All the vehicle's systems remain active when the engine is switched off automatically.
All vehicles (except Mercedes-AMG vehicles): automatic engine switch-off can take place a maximum of four times in a row (initial switch-off, then three subsequent switch-offs). The ECO symbol is shown in the multifunction display after the engine has been started automatically for the fourth time. When the ECO symbol is shown in the multifunction display, automatic engine switch-off is possible again.
Mercedes-AMG vehicles: the number of consecutive automatic engine stops is unlimited.

 

There continues to be debate in the forums over whether the aux battery will charge from a trickle charger on the main battery. My belief is that it won't.

I've studied the schematics and associated descriptions, but you can't tell for certain from those since it's a SAM-controlled function. There's a solid-state switch controlled by the SAM that determines whether the aux battery is connected to the main battery and its associated charging circuits. The logic described in the shop manuals only describes connection during (1) the start phase of eco stop/start and (2) when the engine is running (along with several other conditions) and the SAM determines that aux battery voltage is low.

The load on the aux battery is very small other than when it's handling the start phase for eco stop/start. It should be able to go without a charge much longer than the main battery. If you drove the car once every couple of months it would probably be enough to keep the aux battery healthy. The shop manual states that a trickle charger should be used only if the vehicle will be stored for 6 or more weeks. It only talks about charging the main battery.

 

Guys thank you for responding.

Update: today I took my main battery to Advance Auto Parts to get tested. Battery passed with flying colors.

Then I went ahead and removed the auxiliary battery. What a pain in the neck!

Then I took the auxiliary battery to Advance Auto. It failed the test. No reserve either, which the guy said means it’s completely useless.

I went to Mercedes and bought a new auxiliary battery. (Mercedes part # 2115410001, $164.00 list price. I negotiated it down to a price comparable to Internet parts pricing and the parts guy was happy to come close.)

I will install both batteries later, put the trickle charger on, and see what happens.

I can say one thing: if the trickle charger goes back to 100% charge and float status, then we would have a serious debunking scenario: the theory about the auxiliary battery not being topped off when the main battery is trickle charged would have to be discarded.

(Remember: prior to today, trickle charger wouldn’t go to 100% as it always did in the past. I noticed the past few days that it was acting up.)

Let’s see.

Edit: both batteries installed as of 6pm Eastern, trickle charger plugged in. Hopefully when I wake up tomorrow I will be reporting a solid green light which means 100% charge.

 

Can you please post a copy of the above mentioned schematics. I have not seen one specifically for the SL but have see others (IIRC Varta (MB OEM) here has an example) that imply that the Aux would get topped via a relay when voltage dropped.

 

Here's the relevant pages from the manual. One is an overview and description of operation and the other is the specific portion of the schematic related to ECO start/stop. You'll notice that they call the "Aux battery" the "ECO start/stop function additional battery". I prefer Aux battery myself and will stick with that name.

The overview describes the conditions for which the system will charge the Aux battery. The description is located at the bottom of the document. The conditions described always include a running engine. I guess one way to confirm this once and for all is to locate Relay K114 and see whether it's closed when the ignition is off and a trickle charger is attached to the main battery. If it's not closed then charging current cannot reach the Aux battery, at least according to the schematic.

The schematics are large format and are intended to be printed and then page pairs cut and pasted together.

 

Update.

Fail.

Upon checking this morning, my trickle charger is warm and does NOT indicate 100% charge and float status. It doesn’t even indicate the flashing sequence for 75%+ charge, which is the last step before the 100% indicator. Please note- the trickle charger is good. I swapped to another which I tested yesterday on another car.

Main battery: verified good upon test yestersay
Auxiliary battery: swapped with new yesterday

So now what? I need to pay the dealer to go digging around for a load on the battery?

Please note: under normal and previous circumstances with this particular car and this particular trickle charger, 100% charge level and float status were always reached.

 

Sorry I don't have a lot of time today, but thought I would add a quick note until I get time to post something more coherent on this somewhat complex issue. And typing from a phone is my least favorite thing to do. Hope this can help here….

Recently had a friend with the similar issue and had to go down the wormhole. It was resolved by resetting the front SAM and replacing main battery (which was only 1-1/2 years old). From here, they are now only charging from under the hood terminals. (Their Walmart battery tender was suspect too). I just had them buy a CTEK and so far so good. For what it’s worth, I used to charge my battery from the trunk and have also moved to under the hood terminals.

One of the reasons Mercedes recommends charging from the posts under the hood is the B95 Battery monitor and front N10 SAM (and other components) work in conjunction to maintain the “charge balance” of BOTH batteries. The N10 SAM decides on when and how BOTH batteries get charged.
And also be aware, when the car is shut off, the "auxiliary battery' is disconnected (via K114) from the circuit so it won’t charge. Also be aware charging with terminals connected to the battery directly (or other sources) will bypass either N10 or B95, or both.

Under operating condition, think of it like running water, i.e. it’s directional. Alternator supplies power, goes to front SAM, where, batteries, alternator and a myriad of other factors are being monitored. N10 SAM then decides where and when voltage is supplied to on BOTH batteries. The system controls the output of the alternator under acceleration to increase power and/or save gas by reducing the alternator load and conversely (when needed) increases charging when coasting to increase engine braking.

Here’s an example of how the SAM records data.
​​

 

I think the fact that your Aux battery was totally dead is a clue. I have a 2013 SL and still have the original Aux battery. Something is drawing down your batteries. When the car is off the Aux battery is isolated from the main system and is only connected to the Ignition module and the transmission module. Unless, of course, the two relays that isolate the Aux battery are stuck closed. Overall, it would not be good for the two batteries to be stuck in parallel since they're of vastly different capacities.

For starters, I suggest taking a look at the docs I just posted today on this thread.

 

Sure sounds like a parasitic draw on your battery(ies). I sure would like to see confirmation on how it is or is not possible to keep both batteries maintained with a single maintainer. I do have a pigtail on my aux battery but have not been using it assuming that both batteries were being maintained by using a pigtail on the main battery. I'm fine with using the jump terminals under the hood if this is, in fact, the only way to maintain both batteries.
Standing by.

 

Thanks for your response.

Results to follow once I have them.

My thing is this- nothing changed in the car. Parasitic draw makes sense- but how does that just happen on its own without any change in vehicle equipment, accessories, etc? Would love to peak into the crystal ball and know what could possibly be drawing on the battery.

 

Did you by any chance install a radar detector, audio device, camera or anything else that might be a draw,?

 

Verify that your charger is correct…. I know you swapped it out, but you swapped it for the same exact unit/brand/model right? Connected thru OBDII?
An actual measurement of the batteries ( internal resistance tester is best) would confirm if they are actually down to 75% capacity or a possible scenario where the B95 battery monitor/N10 SAM is reading them fully charged and preventing what it perceives as an overcharge condition.

 

What you just said makes lots of sense.

I wonder if the SAM parameters are incorrect and therefore preventing charge beyond a certain level.

Currently, I am putting 8 amps into the car via the engine compartment terminals using a newly purchased intelligent battery charger which has selections for auto mode, AGN as well as numerical charge levels. Fyi- it’s been locked at 70% for a few hours.

Question- if I were to put a brand AGM battery by itself on the bench and connect a straight charger at 8 amps or a trickle charger- would it reach 100% charge/float status?

 

Since my auxiliary battery was toast, I wonder if it caused a relationship problem between the batteries which caused the sam to act up or intervene. Note- the car has not been started since I installed the new auxiliary battery yesterday. Does any of that make sense?

Maybe I should start the car, drive it a bit, and let eco start/stop work out a bit.

Thoughts?

 

It’s good you have two battery chargers. So the Deltran Battery Tender Plus (OBDII connection) shows 75% charge level as well as the new one you just purchased?
If you isolate the battery it will obviously charge independently. The problem is it will reset some components in your car if you remove it for any length of time, so be careful. I would try connecting to to the battery directly in trunk and see what the charger reads. Connecting it directly to the engine compartment will route charge though the SAM, which is proper, but in this case we’re using the charger as a diagnostic tool. Also measure voltage if possible. My guess is your battery is probably already at 100%

 

For sure👍
All these components need time to calibrate.