steve sl550

Hi Randall,

My understanding (anyone pls correct if I am wrong):

1)AGM trunk battery should read approx 13V when fully charged so first measure the trunk battery with the CTEK connected but, in the "off charge state" (once the CTEK shows fully charged)........V is...............

2)Measure the V at the Aux battery with the CTEK still connected but, only doing its trickle charge and record V..............with the CTEK connected and having put out its full charge and now in trickle mode V should be 13V+

3)Remove the CTEK and measure the trunk batt V.............V s/b close to 13V

4)With the CTEK removed measure the Aux batt V.........with the CTEK removed the Aux will quickly drop to the 12.9 range that you are already seeing that indicates the Aux is in a full charge state.


Bottom line from what we have seen so far from others who have posted it seems charging through the trunk batt is the way ti insure the Aux is charged
See posts 122, 119, 111 (said he got Aux batt warning after long storage with CTEK connected to underhood terms but, it charged up only after starting/running the car with the alternator charging ) and 109.


Be great to finally pin this down so we are all sure.

BUT, is interesting that it seems my MB Hd Tech who said to not use underhood but, to use trunk connection may if fact be correct.

FWIW: I too store the 2018 SL (and previously the 2015) for months w/o use in winter with the trunk as the trickle charging point and have never had a "Aux Batt warning"....also both trunk and Aux batteries are original/ never replaced.

Steve

rmorin49

Well, this is turning into a real learning experience. I have been taking the voltage measurements by sticking the multimeter probes into the pigtail ends. Turns out that when I take the main battery measurements directly on the battery terminals I get well over 13 V with the CTEK connected. With the CTEK disconnected the voltage readings at the battery terminals drop to just under 13V. This is what steve sl550 suggested I would get and he is correct.

I haven't done the same with the auxiliary battery as I will need to remove the panels to gain access to the battery. I hope I can get to the battery terminals without having to remove the darn case around the battery as that is a pain in the A**. I have another pigtail connected to the auxiliary battery and I get the same reading of just under 13V at the pigtail terminals with the CTEK connected or disconnected at the main battery. I want to take a measurement tomorrow at the battery terminals to see if it varies when the CTEK is connected to the main battery and when it is disconnected. Maybe this will help us determine if any current is being sent to the auxiliary battery when the CTEK is connected to the main battery once and for all.

I'm going to leave the CTEK connected tonight and if the weather improves I'll try to access the auxiliary battery tomorrow and take some readings with the CTEK connected and disconnected.

steve sl550

Randall,

Give me your phone # so I can talk you through this.

If you don't want it public send me a PM

Steve

rmorin49

PM Sent

rorywquin

Yea don’t measure V immediately after charging or with the charger connected .

rmorin49

I am terminating my experiment. I have taken numerous voltage readings on both batteries and am getting consistent readings of between 12.9 and 13.2 V. When the CTEK is connected to the main battery and going through the various cycles I can measure voltages as high as 14.7 V. However when I measure the auxiliary battery voltage I get a consistent reading around 12.8 - 12.9 V whether the CTEK is connected or disconnected.

I had hoped I would be able to see an increase in voltage at the auxiliary battery when the CTEK was connected to the main battery but this was not my experience.

I will continue to maintain my batteries by connecting to the pigtail on my trunk battery because I prefer using the quick connect of the pigtail rather than the jump starting posts under the hood. I have had no issues with either battery using this technique.
I was not able to deplete the voltage on the auxiliary battery by just letting the car sit unused. This may be the reason I was unable to detect an increase in voltage at the auxiliary battery when the CTEK is connected. Perhaps there is a relay that only sends power to the auxiliary battery when the voltage drops below a level that I could not achieve.

Perhaps another owner can conduct another experiment to try and and answer definitively how to best maintain both batteries.

​​​​

steve sl550

Hi Randall/All,

As I have posted above (post #126) "12.9 range that you are already seeing that indicates the Aux is in a full charge state."

So I suspect you are correct that the circuitry must recognize this and not further charge until the AUX is "less than" full charge by some ammt.

As far as I am concerned I stand by what I wrote above in Post 126:
"Bottom line from what we have seen so far from others who have posted it seems charging through the trunk batt is the way ti insure the Aux is charged
See posts 122, 119, 111 (said he got Aux batt warning after long storage with CTEK connected to underhood terms but, it charged up only after starting/running the car with the alternator charging ) and 109.
Be great to finally pin this down so we are all sure.
BUT, is interesting that it seems my MB Hd Tech who said to not use underhood but, to use trunk connection may if fact be correct.
FWIW: I too store the 2018 SL (and previously the 2015) for months w/o use in winter with the trunk as the trickle charging point and have never had a "Aux Batt warning"....also both trunk and Aux batteries are original/ never replaced."

I too am closing out on this topic unless someone else has concrete info.

Tx Steve

rorywquin

I disagree.

The only way to put a pin in this is to see a wiring schematic.

My experience and some others here is different to yours. I only ever charge from the jump start points and have never had an issue (as also stated by others in several other posts here) in 5 years of ownership and very low usage of the car.

I’ll also wager that the jump start (+) point has a direct connection to the battery (+) (via the starter motor solenoid).

The fact that MB stated that the jump start points should used for charging and then owners deciding to go into the trunk & unbury the main battery to add pigtails for charging defies logic.

Hopefully someone can come up with the schematic but I’ve spent hours searching for one without luck. My wife’s car is due for a service by our local MB indie and I’ll ask him if he can look it up for me when I’m next there (he worked for MB for 20 years in their workshops before starting his own business).

steve sl550

I'm in for a schematic too BUT, with a functional description as well