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Thanks, hornethandler. What setting do you use on your CTEK for the auxiliary battery? I use the small battery setting for my lawn tractor. I plan to try and access the auxiliary battery in a couple weeks. If the CTEK pigtail is too short, I will use one for a Battery Tender Junior. I believe it will work OK with this size battery. What do you think? I have 2 CTEKs and use one of my them for the main SL battery and the other one for my lawn tractor. I guess I could buy another one.
I’m using the CTEK MXS 5.0. It does small and large 12V lead acid and AGM batteries. It also has a selectable recondition cycle.
For the Aux battery I fashioned a jumper cable with magnets. The positive goes to the positive battery terminal, and the negative goes to one of the mounting screws that secures battery/circuit breaker cage. The cages’s top center fastener goes to ground. Just sand the paint off the front of the fastener. I use the "small battery" setting for the Aux battery.
Here’s a some pics
Last edited by hornethandler; Jul 14, 2020 at 11:55 AM.
I’m using the CTEK MXS 5.0. It does small and large 12V lead acid and AGM batteries. It also has a selectable recondition cycle.
For the Aux battery I fashioned a jumper cable with magnets. The positive goes to the positive battery terminal, and the negative goes to one of the mounting screws that secures battery/circuit breaker cage. The cages’s top center fastener goes to ground. Just sand the paint off the front of the fastener. I use the "small battery" setting for the Aux battery.
Here’s a some pics
I'm trying to understand how these magnets work...it doesn't seem very secure, somehow. I was going to put the eyelets underneath the terminals altogether, before I noticed that the pigtail was kind of short when trying to route the wire down and around the outer side of the battery so it would come out at the lower-outer corner of the front panel. I didn't want the pigtail to exit in the middle of the top of the front panel.
I'm hoping my pigtail will be long enough to exit on the floor behind the passenger seat. If it's not long enough I need a Plan B. I plan to attach the pigtail to the terminals under the bolts.
I considered a permanent installation as well, but didn't like the idea of wires that could possibly get pinched and shorted, especially in an area you may store stuff from time-to-time. The magnets provided a simple solution, and the bonus was not having to remove the cage to access the battery terminals.
I considered a permanent installation as well, but didn't like the idea of wires that could possibly get pinched and shorted, especially in an area you may store stuff from time-to-time...
If I do elect to proceed with this project, I'm going to be very careful about the routing/length of the pigtail, for this reason. That's why I didn't install the one that came with my charger...it was too short to route properly.
I just verified that the three fasteners securing the bottom of the cage (not including the horizontal fastener on the front of the cage) go to ground. Using one of them may give you the length you need without modifying the existing pigtail. I’m sure you know this, but be sure to sand the black finish off the fastener as well as giving the threads a light sanding.
I'm hoping my pigtail will be long enough to exit on the floor behind the passenger seat. If it's not long enough I need a Plan B. I plan to attach the pigtail to the terminals under the bolts.
Are you experiencing any problems having TWO CTEK's connected to your R231 at the same time?. I love my CTEK, currently, only have one connected to the main battery. Would like to install a second CTEK similar to your installation on my Auxilary battery but my local MB service is trying to tell me it will "mess with the electronics" and void the warranty.
I have connected both of my CTEKs at the same time, one to each battery and both functioned as expected with full charge occurring within 24 hours. I'm no expert but am not concerned that they will have any adverse effects on the "electronics".
I have connected both of my CTEKs at the same time, one to each battery and both functioned as expected with full charge occurring within 24 hours. I'm no expert but am not concerned that they will have any adverse effects on the "electronics".
Thank you. I just wanted to confirm both chargers actually worked in tandem and you were not experiencing any electronic gremlins. MB corporate is on the record saying dual charging will definitely cause problems and will void warranty; in my specific case, my dealer refused to install the quick connect charging cables on my auxiliary battery during replacement of the aux battery.
Interesting. My dealer recently accessed my auxiliary battery while diagnosing another issue, inoperable Air Scarf, and must have seen my pigtail. At the time I did not know it was there. They knew I had a CPO warranty because it paid for the repair to my AirScarf, some sort of controller. If I have an "electronics" issue perhaps I will remove the pigtail?
Has anyone seen anything in the owners manual or official MB guidance that says you cannot use a battery maintainer on the auxiliary battery?
I suspect, so many people are either not driving their cars at all, or are driving so much less than normal, an large number of owners are finding their batteries have died due to non-usage. If MB is suddenly deluged with a larger than normal number of dead battery claims, under warranty, someone in MB corporate may have figured out a way to avoid replacing the batteries under warranty completely - just blame it on battery chargers that were installed by the owner. The claim that modern chargers are too powerful, cannot be regulated to multiple amp settings and cannot cease charging completely once the charger senses the battery has been fully charged are totally specious.
Mercedes actually sold their own privately branded model of a CTEK charger until just recently. Interestingly, the MB CTEK was a much older model that did not offer the option of high and low amperage while charging - it would operate on high only. A higher then necessary amperage might indeed cause harm to a battery when charging as would excessive overcharging if the charger did not have the ability to stop charging once the required charge was sensed.
COVID is going to be with us for a while. As long as COVID persists, people will continue to drive less. Less driving, more batteries will fail because they are not being charged, and will need to be replaced. MB would much rather charge us $450 to replace dead batteries than install them for free under warranty. If MB would simply test modern chargers, such as the CTEK 40-206 and approve it for use on Auxiliary batteries, owners would no longer have dead batteries and MB would avoid the cost of replacing otherwise perfectly good batteries under warranty. It would truly be a WIN WIN situation.
I don't know about a warning in the manual, but when I spoke to MB national corporate support on the phone this afternoon, they read me a very detailed statement about the evils of putting a charger on the Auxiliary battery and that doing so would void my warranty. It would appear your own use of 2 chargers (one for the main battery and one for the Aux) had not caused any catastrophic electronic failures - unless your Air Scarf issue was actually a result of the charging. How would we know?
I would hope others who are using dual charges would post their experiences here.
I have 2 CTEK Model 4.3. I use the small battery setting for the auxiliary battery and the larger battery setting for the main battery. I, too, would like to hear from other owners especially any that have had any issues associated with using a battery maintainer. I understand that there is a big difference between a maintainer and a charger. You can ruin a good battery with a charger if you are not careful but the modern maintainers are designed to slowly bring a battery up to full charge and keep it there without any overcharging.
I usually keep my CL and SL stored in my garage from November to April and I do not charge the starter battery, never had a problem, the previous SL 500 was stored during the winter for more than 10 years, of course I had to replace botH batteries because of their age but that was about it. I intend to do the same with the CL and the actual SL (which I had it for three years already) And by the way I’m using the CTEK chargers.
Just my .02
I'm trying to understand how these magnets work...it doesn't seem very secure, somehow. I was going to put the eyelets underneath the terminals altogether, before I noticed that the pigtail was kind of short when trying to route the wire down and around the outer side of the battery so it would come out at the lower-outer corner of the front panel. I didn't want the pigtail to exit in the middle of the top of the front panel.
Ha, ha. I see what you did there...
I charge from the jumper points - never had a problem with the aux battery.
Now that's a question I was going to ask. Can you charge the battery using the jump points in the engine compartment. (I know that it's just been answered) But would this method charge the Aux battery at the same time. Please forgive my ignorance on this matter as I have said previously this is the first car I have owned that has two batteries.
My understanding is that the auxiliary battery cannot be charged using the connections under the hood. The former owner of my SL installed a CTEK pigtail on the auxiliary battery so I am using 2 CTEKs to maintain both batteries whenever it sits unused for more than a few days. I do note that it takes much less time for the auxiliary battery to reach full charge compared to the main battery. I am using the "snowflake" mode setting for the auxiliary battery. This is also the setting used for AGM batteries.
Now that's a question I was going to ask. Can you charge the battery using the jump points in the engine compartment. (I know that it's just been answered) But would this method charge the Aux battery at the same time. Please forgive my ignorance on this matter as I have said previously this is the first car I have owned that has two batteries.
I’ve verified on my 2019 450 that the the aux battery does not charge from the “jump points” under the hood.
Also, it’s generally understood, but not completely verified, that when the vehicle is off no power is used from the aux battery.
I’ve been monitoring the aux battery voltage and that does seem to be the case, i.e., I check it at shut down and periodically when the car has been sitting for several days. After a week it only self discharges about a mill volt or so. Since it appears no juice is used when the vehicle is off, and because AGM battery self-discharge is so low, I’m wondering if trickle charging the aux is even necessary unless you’re planning on extended vehicle storage.
I have been told by the Shop Foreman @ my dealer that the best way to keep a maintenance charge on the SL is via the main battery in the trunk so my trickle charger is connected to the main batt posts.
I suspect that the jump posts in the engine compartment bypass the main and aux battery and likely connect only to the starter ckt thereby protecting the main and aux batteries from an overcurrent surge when jump starting.
I may be wrong but, if someone has a circuit schematic maybe we can figure this out.
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