It's just that someone suggested that if you connect your charger directly to the main battery terminals, a SAM will monitor the charging process and also route charge to the aux battery if/when required...his implication being that connecting to the under-hood points instead would somehow remove the SAM from the process, which would prevent any charge from ever reaching the normally isolated aux battery. I'm picturing the same as you: a fat connection running from the under-hood jump point(s) to the main battery, so I don't understand how connecting under the hood could negate the SAM's participation either. There has been enough MB-sourced info on aux battery threads here to suggest to me that the aux battery does not charge when I charge my main battery. Like probably everyone else here, I sure wish there was more clarity on this issue.

Actually, I was questioning how the CTek's "smarts" would interact with those of the SAM...and if, as suggested above, the SAM is somehow in parallel with the main battery, then it would not necessarily be bypassed from the aux battery's viewpoint.

My only concern regarding the SAM/CTek issue was this: if there is a SAM that watches and senses charging voltage/current, could it somehow be "freaked out" by the rather convoluted and sophisticated multi-step CTek charging regimen? The CTek goes through seven or eight different programs/steps, each of which has different voltage/current characteristics (square-wave, pulsing, more voltage with less current, less voltage with more current, etc.) depending on what the immediate needs of the battery are...which in itself suggests the CTek is continually doing its own evaluations on what to do and for how long on each of its steps on the way to "full" charge. Might not this whole process collide in some way with the evaluations taking place in the SAM? And if so, who wins and who loses? What does the CTek think if/when the SAM ostensibly switches the CTek's load from the main battery to the aux battery, and the CTek suddenly has to start all over again while running a "big battery" charge, only now into a small battery...and what does the SAM think when it's exposed to the CTek's various different charging profiles, etc.? Maybe I'm overthinking the whole thing and it just simply all works...IDK. You and I have been just fine connecting our CTeks under the hood. Our only difference is that, based on the documentation that I've seen here, I have assumed that my occasional long drives are all that is responsible for my aux battery's continued good health...whereas, you are satisfied that your under-hood charging is responsible for your aux battery's continuing good health. Understanding what is actually happening would be nice, but that's not really necessary for us to continue enjoying our fantastic cars, is it?

 

It would be interesting - I’ve scoured the www and have been unable to find moredefinitive information.

 

Great VIdeo and very well presented. I did this procedure some time ago having watched this. I now keep the relevant torque driver in the boot / trunk ( I'm in the UK ) in case I need to access the fuses when away from home.

 

I just did this AUX-Battery replacement on my 2013 SL550. I could not remove the battery "cage" without removing the center console bezel. But I could not remove the bezel (at least not completely and the force required was so great I stopped for fear of breaking the roller-door housing or the bezel). So I taped off the trim pieces for protection and carefully cut the tabs away. After the replacement I noticed the date code on the battery was 31-12 (August of 2012, 9-1/2 years old).
I would suggest that these cumbersome tabs serve a purpose during manufacturing/assembly of the car, rather than a nefarious intent on the the part of MB. But, who knows....

FYI

 

I use a CTEK MUS 4.3 on my SL especially in the winter months when it may sit for several weeks at a time without being driven. I don't believe in starting it in the garage. I let it sit until I can drive it for at least a 30 minute and 10 - 20 mile course.

The charger has 4 modes: small battery, normal battery, cold weather and recondition.
The normal battery mode says it is for wet, Ca, MF, GEL and many AGM batteries.
The cold weather mode says it is for charging at low temperatures described as -20 degrees C to +5 degrees C and for power AGM batteries like Optima and Odyssey.
My garage will rarely get below 10 degrees F and neither of my batteries are an Optima or Odyssey. The main battery is a Varta AGM and the auxiliary is made by FIAMM, I think it is also an AGM. Both are vented properly.

My question is: Should I use the Cold Weather mode during the winter months and the normal battery mode during the warmer months of the year? When I look at the chart that describes the various steps in the charging programs I do not see any difference in the charging voltages during the 7 stages of each program.

Any comments or recommendations? Thanks.

 

"Should I use the Cold Weather mode during the winter months and the normal battery mode during the warmer months of the year? "

No the winter mode is for AGM. Only use that. The algorithms for duration, voltage and current for AGM are different to regular lead acid batteries.(I actually emailed CTEK(about 5 years ago) to confirm this

Your battery will almost certainly be regular be AGM (Optima and Odyssey are a type of AGM but different construction) . I use the “winter/AGM mode” on mine & wife’s car all year round (5 years now) without any issues.

 

Ditto - 5th winter with it now (I use MB recommended jump start points under the hood).

 

No issues with your auxiliary battery? How old is it?

 

I will let him answer about his battery but mine is a 2013 and had it replaced by the dealer in 2020, the battery was still good, so, I would say, at least 7 years you shouldn't have any problems, however this is about a OEM one, not so sure about other brands....

 

No issues at all.

Just about 5 years old. I got the car new in April 2017 only done 17,000 miles (so it stands a lot).

 

Did you use a maintainer on your auxiliary battery? What about the main battery and if you did, where did you connect your maintainer(charger)?

I started getting an auxiliary battery warning a few months ago. Replaced it with an OEM battery with a 2021 date code. Date code on the old battery was 2014 so it was most probably the original one. I think the previous owner did use a maintainer on it because there was a CTEK pigtail attached directly to the auxiliary battery.

I'm still trying to confirm if you can maintain both batteries with one maintainer and if so where do you attach it.

 

Thanks. You are using a single maintainer attached to the charging points under the hood?

 

I'm only using a CTEK maintainer attached to the battery in the trunk, nothing on the auxiliary.

 

Thanks. That is what I am doing now as well. I hope this method also keeps the auxiliary battery charged.

 

My SL has stood for a month. I started it 2x last week (reverse out garage and drive it back in). Yesterday was a lovely sunny day so we decided to head out for lunch at one of our favourite gastro pubs.

1) Car has been on the CTEK via the jump start posts for the month.
2) When I drove off the ECO light was orange (I put it down to taking the car out the garage last week) but changed to green within about 5 minutes. (FWIW I fitted a SmartTop which has a function to disable the ECO so it does not get used by me).
3) Usually if the car has not been left on the charger, it would take at least an hour of driving before ECO turns green.

My conclusion:
1) Charging as recommended by MB at the jump start points is fully effective - In 5 years of low usage ownership (low mileage - 17k) & using the jump start posts to maintain the battery / batteries I’ve had no problems with the car
2) I don’t know for certain if the AUX battery is getting charged or not, but it certainly is not detrimental to the AUX battery.
3) There is no benefit charging connected directly to the battery in the boot/trunk
4) I see no need for wiring pigtails to the AUX and charging it separately. These are VRLA batteries & are used extensively in applications where low maintenance batteries are needed.

 

Randall,

I believe you were going to check the charge on the AUX battery via the pigtail to confirm that the charger connection to the trunk battery does charge the AUX.

Have you performed the test?

We all want to know results.

Tx

 

Unfortunately, I had surgery and have been unable to work in my garage. I hope to be ambulatory next week and will attempt to determine if the auxiliary battery is being maintained by using the pigtail on the main battery. If anyone has any suggestions as to how to assess this situation please post or send me a PM. I do still have a pigtail attached to the auxiliary battery and planned to see I can detect any current by attaching a voltmeter to this pigtail while the maintainer is connected to the main battery. Does this seem plausible?

 

Suggest to:

1)Check V @ the AUX battery pigtail with charger connected
wait a day with charger disconnected
2)Check again with charger disconnected
With charger connected vs disconnected should see some difference.

Let us know

Tx

 

Will do.

 

I’d wait a couple of weeks with the charger disconnected or start and stop the engine a few times without driving.

 

Hi Randall,

Let us know what you find.

Tx, Steve

 

Took first reading on the aux battery this morning. Maintainer was on the main battery for 2 weeks. Voltage was 12.9. Going to let the car sit for several days and take another reading. If voltage drops I will put the maintainer back on until it cycles back to green and take another reading to see if the voltage goes back up.

 

Checked today and the auxiliary battery voltage has only dropped 0.1V. I'll check again in a couple days. I'd like it to drop to just below 12V. I'll then connect my CTEK to the main battery and see if it brings the voltage back up.

 

Hi Randall,

You plan sounds good.

Look forward to your results so we can put this question to rest.

although.............only way to really see if trunk connection better, same or worse that connecting to the engine jump start location is to:

1) Have your trunk mounted connection results

then

2)Try same scheme you used for the trunk connection connecting your charger to the underhood charging locations and then testing at the Aux connection

Comparing 1 & 2 really only way to close this question out.

Really only way to see if either or both methods provided charging to the aux

 

I agree and will do exactly that. I will also use the under hood terminals with my CTEK to see if these are or are not able to send current to the auxiliary battery. Stay tuned, this might take several weeks since I have to wait before each step for a decline in voltage. I thought about trying to reduce the voltage by leaving lights on but not sure if this will work as they may be designed to turn off automatically. I will do some experimenting over the weekend. I'm hobbling around house after my foot surgery so I have some time on my hands.