rorywquin

But why is it relevant!?. MB say to use the jump start points. Unless it is impossible to access the jump start points and people need to get into the trunk & at the battery itself.

The starter in a car has one wire going to earth and a very fat copper wire coming from the cranking battery to the solenoid on the starter. The solenoid will be activated (by turning the ignition key) which will then close the solenoid and connect the fat wire to the starter. For jump starting you will have a similar fat wire from the jumper post to exactly the same terminal on the starter solenoid.

battery (+)> fat red wire> starter solenoid < fat red wire < jumper start post (+)

steve sl550

Hi Rory,

Yes the "underhood" terminals will allow a jumpstart and charge the main trunk battery; there is no question regarding that.

Question is and reason for Randall's experiment: .......is to determine if charging via the underhood terminals will charge both the main trunk battery and the aux battery.

As Randall has a pigtail connected to his aux battery he will be able to see if the aux battery will charge/or hold it's charge if a maintenance charger is connected to the underhood charge points or only if connected to the main trunk battery.

The head mech at my dealer told me (I have posted his comments earlier with the schematic he provided) that the aux battery is not in the circuit when charging from the underhood terminals.

Be great to get results from Randall's testing to put this to bed once and for all.

In Randall's last post he stated:
"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."

I suggest he do his test BUT, in addition he perform a similar test using only the underhood connections. If he does both tests we can compare the 2 results and see if the underhood connections have any affect on the Aux or if the only way to charge the Aux batt is by connecting to the main trunk batt OR driving the car providing alternator output to the main and Aux.

Be great if Randall can do BOTH tests so we can put this question to bed once and for all.

rorywquin

Will be interesting to see the results of the but I suspect that they will be inconclusive.

Reasons:

1) The charging system will be designed so that the main cranking battery is always properly charged first.
2) I suspect that when the ECO goes on orange it is because the cranking battery is not fully charged so that the engine is not stop/starting with a low charge cranking battery. This is also borne out by this statement from the owners manual because if the engine stop / starts too often, it runs the cranking battery too low."


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2) Aux batteries are VRLA and designed for low maintenance and are thus unlikely to loose much charge while standing and is only used to maintain electrical systems etc while the engine is in ECO stop and prevent surges when the car restarts. There is a mistaken beleif that teh aux starts teh car during ECO stop start events.

steve sl550

Rory
All we trying to find out with this test is if the Aux batt is charged when a charger is connected to the underhood terminals, connected to the trunk battery or either works to charge the Aux battery.

Some confusion if the underhood connections are only for jump start or main/trunk battery charging and will not charge or keep charged the Aux battery.

So.......if Randall performs the following we should be able to find out what trickle charger location/locations do in fact charge the Aux battery.

Maybe a good test is:
1) Take readings at his Aux battery pigtail as follows as a baseline:
a) Reading @ Aux pigtail when charger connected to trunk battery and charger indicates full charge. Reading is...........
b) Reading @ Aux when charger connected to underhood battery connections and charger indicates full charge. Reading is...........
2) No charging for a week at most although probably don't need to wait this long because once the trickle charger is removed the Aux battery voltage shown from the pigtail will drop so Randall should almost immediately see a slight drop from voltage when the trickle charger is in the circuit.
3) Retest again @ a & b with trickle charger connected and if increase is seen note voltage and compare to voltage in 2.

Test 3) will show If theAux is charged from the underhood terms , the trunk battery only or both.

If I had the Aux battery pigtails on my car like Randall has this is what I would do to settle this question.

Steve

rmorin49

I'm going to take voltage readings tomorrow on both batteries. If either or both are below 12.5V I'll connect my CTEK to the pigtail on the main battery and allow it to cycle through to a fully charged state and then check voltages on both batteries. I'm hoping the auxiliary battery has at least dropped a bit from the original reading so I can see if connecting to the pigtail on the main battery will increase it.
If it does not I'll try connecting to the under hood charging points and see if that increases the auxiliary battery voltage.

rorywquin

Hi Steve,

I suspect that the Aux is always disconnected from the cranking battery / charging system until the cranking battery is fully (or getting close to) charged and the aux NEEDS charging (bearing in mind that this (aux) battery type was selected because it is low maintenance and can tolerate a deep discharge). Not knowing how the batteries are connected via relays etc is never going to give a conclusive answer.

I've done a lot of remote touring (most recently in the Australian outback during the 7 years I lived there ) & my LandCruiser 200 series had 3 batteries under the hood ( 2x cranking batteries plus the auxiliary I had fitted), plus an array of batteries in my off-road caravan (designed for remote bush camping which was equipped with a mains charger, solar array and capability of being charged from the car. In all instances the LandCruiser cranking batteries had priority over charging any of the other batteries. I got my alternator clogged with mud in the SImpson Desert and was able to limp to the next town by jumping various batteries and using my CTEK for charging batteries from a Honda generator over 3 nights & days (just saying, I've been around the block a few times with car batteries ).

If the aux battery does need maintenance charging, MB would have designed this into a $100,000 car without the need for (aftermarket) pigtails or direct connection to the cranking battery. However, people need to satisfy themselves.

Good luck whichever way you choose to go. I look forward to the results but .......

Take care. We have enough stuff going on with Covid & Ukraine without getting cranky over car batteries.

steve sl550

Hi Rory,

Not getting cranky ; I see it as an academic exercise.

Always enjoy your posts and comments but in this case I suspect my MB tech mgr may be correct regarding the underhood connections not charging the Aux but, connected to the trunk battery will.

May just be that MB didn't want high charging current from the alternator to run to the small Aux battery so designed it to be charged through a regulator via the main battery?

Sure be great to have a real full schematic, if we had one we would know for sure.

My MB guy only was able to give me a partial he was using to try to explain why would not work to chg the Aux from underwood BUT, who knows he could be wrong (he no longer with MB as he is now an IDDY in Florida !).

We all relying on Randall's testing so we know for sure.

Steve

rorywquin

Wasn’t implying you're getting cranky. just a general comment about the state of the world. Yea I’ve searched for a diagram. The closest is the one on the Yuasa site. Aux will deffo be separated by a relay. If not the one with the lower voltage would drian current from the battery with the higher voltage.

ToeKneee

I personally think that the front charging posts are connected to the rear battery only and not the auxiliary battery at all so when they are used to charge the car only the rear battery gets charged. Once the car is running, the electronics will check the state of the rear battery and if it needs charging it will charge it, then once it's charged will direct the alternator to charge the aux battery if it needs it. If the main battery is keep charged up via an external charger/maintainer then the aux battery will get charged sooner once the car is running.
A simple test would be to disconnect both the rear battery and the aux battery and see if there is any continuity between the charge post and the positive terminal on the rear battery (I believe there will be) and any continuity between the post and the aux battery (which I believe there won't be), that'll give a definitive answer.

rmorin49

Not much to report. The voltage reading on the auxiliary battery is still at 2.89V. I opened and closed the top a couple times using the remote and without starting the car. This has perhaps help reduce the voltage of the main battery. It measured 2.3V. I don't know how to deplete the auxiliary battery other than not driving the car. I'll let it sit for several more days and then connect my CTEK to see if I can increase the voltage on the auxiliary battery. Does anyone know what is considered a full charge reading for the auxiliary battery?

Optome

My recent experience tends to to add credence to ToeKneee's post. Midwest weather finally warmed up enough to take the SL off of the CTEK after a long nap. Immediately noted the Auxillary Battery Malfunction warning upon start up. After a good warm up and a 50 mile drive the auxiliary battery warning did not return on the next start up. So far so good.

rmorin49

This is why I am trying to determine if connecting a maintainer to the main battery does, in fact, also maintain the charge on the auxiliary battery. I know many SL owners store their car during the winter months or like me not drive it for weeks at a time. All I do know is that the original owner of my SL installed a pigtail on the auxiliary battery so I assume they used it. When I discovered this pigtail I started using it as well. I did replace the auxiliary battery a couple months ago when I got a warning but the date code showed it was almost 8 years old. I don't know if using the maintainer helped it last that long or maybe that is the typical life of this type of battery.

steve sl550

Optome,

Question: Was your CTEK connected to the underhood terminals or directly to the trunk battery.

Pls advise.

If was connected to the underhood terminals and the Aux battery was discharged lends credence to fact that the underhood connections do not charge the Aux battery.

We all want to know.

Pls advise.

steve sl550

Optima pls see my post above...looking for your response as to where your trickle charger was connected.
Tx.

Randall,
The 12.89 you show for the Aux batt is normal for a SLA (sealed Pb acid batt) when charged and not connected to any charging supply.
When a charger is connected to the Aux it should show approx 13.7V BUT, when the charger removed V will drop to 13.2/13.4 then after a short time to the 12.8/12.9 you are seeing so it seems your Aux was getting charged and is reading correctly for a charged Pb acid batt.
BUT
If your trunk battery is down to 12.3V without trickle charger connected be interesting to see what you get if you connect the trickle charger to the trunk battery, monitor the trickle charger until it shows "charged" then read the trunk batt V...........?
THEN
Go to the pigtail of the Aux battery and with the trickle charger still connected to the trunk battery the Aux battery should read MORE than the 12.89 you are now showing if in fact the Aux is being charged from the trickle connection to the trunk battery. V at the Aux if charging will be something over 13V if charging to the Aux is in fact performed by charging the trunk batt.

Good to know that the higher Aux batt V of 12.89V is not flowing back into the partially discharged trunk batt 12.3V so obviously the V travels only from the trunk batt to the Aux as it should.

While we wait for OPTIMA to answer my question as to where he had his trickle charger connected that allowed the Aux batt to discharge (suspect it was connected to the underhood terms) you can replicate the test above connecting to the underhood and if when connected and indicating fully charged the Aux batt does NOT go above 13V the underhood is NOT charging the Aux


Be great to finally figure this out

Optome

Steve sl550,

My CTEK has always been connected to the under the hood terminals. As ToeKnee suggested, with a fully charged battery, it didn't take long for the engine to charge my auxiliary battery.

steve sl550

Tx Optome.

So....based on what Optome has said seems that the underhood connections do not allow trickle charging of the Aux batt BUT, when the car runs and the alternator is in the circuit it will charge the Aux.

This is what the Hd Mech @ my MB dealer told me as I have reported; he also said connecting the trickle charger to the trunk batt will charge/keep charged the Aux.

Nail in the coffin regarding this (one way or the other) will be when Randall completes his testing.....see my post #114 above with test suggestions for Randall to use.

Interesting

rorywquin

Agree, there will always be continuity 'jumper>starter<main battery'. However, the above scenario implies relays (which there are - referrred to as “contacts" in the diagrams below) keeping the Aux isolated until the main battery is charged which negates your test for jumper>auxillary as the aux will become isolated for a low charge on main battery (i.e. by disconnecting the main battery the relay will isolate the auxillary so that the main battery gets charged.). There has to be a way (contacts / relays) keep the two separated or else the one will discharge the other until there is no potential difference between them. Same as putting a flat rechargable battery in your flashlight with other charged batteries.

rmorin49

I'm going to connect my CTEK to the pigtail on the main battery in the trunk and leave it until it cycles through to a fully charged state. I will then check the voltage on the auxiliary battery using the pigtail that is connected to it. I should have results by tomorrow around noon as it usually takes about 24 hours for the CTEK to go completely through all 6 steps. Stay tuned.

crconsulting

I use this method. This is how I was always TOLD both batteries would charge. I've never had my batteries run down but drive pretty much all year, as long as it doesn't rain. So can't add anything here.
I applaud everyones scientific approach though. 👍

steve sl550

crconsulting

Have noticed that you are quite knowledgable on our cars and we appreciate your many responses to our issues.

Mind if I ask if you are a MB Tech or Tech Mgr @ a dealership?

crconsulting

Retired now. Haha I have a somewhat storied past.... And I don't want to sidetrack this thread too much.
But I look at it this way. If I can help with any of the issues and frustrations people experience from service, through my practical knowledge, and can add to the online knowledge base, then it's my way of giving back a little. I have found great info on this (and other) sites due to others who have spent time and laid out great tutorials. There ARE good shops, but it's hard to get consistency from larger shops when you deal strictly with a service writer who has never worked on cars.

I worked and trained at Mercedes but left long before the R231's. Still have a few contacts and try to stay current. Worked and ran other challenging automotive and machine shops for many years. Porsche, BMW factory and others in the industrial machine tool sector. Grew up in the trades, and in the end, it's part of my DNA. Like most here, love cars, but working on cars is physically demanding the older you get. I have a couple of friends with R231's and W222's (and a few others) I still service, but thats as much as I can take now. And only really do that, because they've been burned so badly by incompetence in the industry. I feel bad for them...

btw I didn't retire in the automotive/machining field though : )
Being a nor-cal native. That silicon valley industry $$$$ magnet was too strong...

tx170754

Guys,
Just my .02 on this "hot" subject. I always keep my SL's ( previous model and this one ) stored during the winter time, starting about 2005. I'm only using the CTEK attached to the main battery terminals in the trunk ( not so sure if it makes a difference or not ) but one time during those years I've tried to charge the aux battery after a couple of months of storage, the battery was completely charged.

rmorin49

Update: My CTEK is at stage 4. I should have a green light tomorrow morning. If I do I'll post the voltage readings of both batteries.

rorywquin

If the AUX was at a level of charge where it did not changing, there is going to be no change. Let’s see what tomorrow brings.

rmorin49

I just checked the voltage on both batteries. The CTEK is showing a green light which I believe is supposed to indicate a full charge.

Auxiliary battery: 12.97 V
Main Battery: 12.5V

Both of these readings are slightly higher than they were before I connected the CTEK to the main battery yesterday. That said, my multimeter is a very inexpensive one I bought at Harbor Freight so the readings may not be exactly accurate. I have no idea how to verify the readings.

I put the CTEK back on the main battery and I will let it go through another cycle and check the voltages tomorrow.

A couple more notes:

1. My auxiliary battery is very new. I replaced it about 4 months go with an OEM battery with a date code in late 2021. I can't remember what month.
2. My main battery was in the car when I bought it. I need to look at the date code to see if it is original. I will do that tomorrow as it turned very cold here with sleet and freezing rain. Even though the car is in my unheated garage I am still recovering from foot surgery and don't want to schlep around in a cold garage. The weather is supposed to be better tomorrow.