NickTH

I Use the CTEK battery tender on the front/under the hood main battery, do I also need to hook it up to the aux battery in the trunk separately also?
or does charging the main front battery takes care of both?

Thanks,

KEY08

You would need to hook the Aux battery up separately if you plan to charge it.

kajtek1

Why do you need battery tender in first place?
You do know that some of them are famous for killing batteries on long run?
The aux battery is charged by special module and per my knowledge is disconnected from the car in park.
I would not worry about chargers unless you park the car for more than a month.

cetialpha5

I wouldn't leave it connected for long periods but if it's just for the day, that's a good idea for the aux battery as they seem to fail quite frequently and that's probably from it not getting a full charge. A fully charged battery will last longer than one that never gets fully charged up.

LALALAND

Charging from the front will not take care of both, only the main battery. As for the aux battery in the trunk, I took it off then charge them out of the car. Someone mentioned to do it like that in the forum before. About 2 hours with a ctek charger which I also have for the aux to get to stage 7. Does it prolong the batteries? Probably so as I haven't see the aux battery malfunction message yet(knock on wood). My ECO mode doesn't work unless driving long distance on freeway (at least 20 miles). This might signal the battery are near the end of life. Note that I have not try the Recondition mode on the charger with AGM batteries yet. Not sure if they work or not or have any harm.

NickTH

Does anyone knows why we need to remove the aux battery from car trunk in order to charge it?
Why would charging it while still left connected in trunk be harmful?
Thanks,

kajtek1

While using uncontrolled charger - battery can spit corrosive gases and acids - beside risk of sparks when connecting.
You don't want it in trunk with no bottom vent.

GFBR129

My car will be under a cover in the driveway for 3 months. So i am worried that the battery will die, and all the electronic will have problems. I could run the car once a week.

stug477

I have a 2017 S550 at my Florida condo with 9,500 miles on it. I got it from my cousin when her lease was up in October of 2020 with 7,611 miles. It was parked with a Stanley battery maintainer connected to the main battery and because of COVID I had not been back to Florida for two years. When I did return the main battery was perfect however, I received message "Auxiliary Battery Malfunction". Replacing it was a project so taking it out to charge isn't feasible. The car was leased in October of 2016. Over six years with minimal use is pretty good battery life for one not maintained. I could have easily connected maintenance leads to the battery when I replaced it..

DFWdude

In thread after thread on this forum, opinions continue to vary on whether charging the main battery also charges the auxiliary battery in the trunk.

The auxiliary battery in my trunk is OEM from manufacture (2015) as far as I know. I'm on the third main battery up front, and I recharge it every 6-8 weeks or so, when the car gets minimum use. I've not gotten the Auxiliary battery notice, yet.

Left Coast Geek

fyi, thats not a w212 E class. I believe its a W222 S class.

GFBR129

Thank you, I have been using a battery tender now and it is working great.

CaliBenzDriver

4-- Solution optimized:
okay, we've seen we need to charge both batteries disconnected.
Go out and disconnect them like so :

-> Simply disconnect MAIN GND fat brass jumping nut. Connect charger to battery side.

-> Disconnect AUX both sides (M10 screws) -Connect to battery side.

> Outcome:

• fresh computers on their best behavior.
• fresh batteries like new.
• No strain on car electrical system.
• Nearly $0 expense!
• Empowered ownership.
• Cars VIP Modules like to be discon

I call that pretty good value.... thanks MBWorld forum ✌️

+++++----
Single charger...:

• ​​​​​Only float charge MAIN
• disconnect AUX

--++++++--

1-- The MAIN battery supplies power to some electronics (SAMS) while it is in low power sleep mode.
Even on cars without accelerated battery drain (sleepless modules), this eventually runs down the battery charge level and its output voltage.
This is where a battery float charger pays for itself!

2-- The AUX sits there doing exactly nothing and therefore its nominal charge can last a lot longer.
It's a lead-acid AGM, it does not fair well to sulfation - It also needs to be floated for extended storage.

3-- BEWARE: Dual Tenders!!!
The Smart tenders involved here are connected to what British call "the Mains" - That means the commun DC GND gets referenced to "AC Mains" without a galvanic insulation through a transformer.

I would not connect two chargers to car GND at the same time... this is when I get my chicken legs!

(This non-isolation topic would become @S-Prihadi inputs )
[two batteries connected to 2x modern float chargers with a shared battery side GND connection.... ? What happens on the AC Side? how much current?]

kajtek1

Lucky guy.
I remember my aux battery popping the light at 6-7 years mark.
But do you use ECO stops a lot? I do as we have 5 miles street drive to freeway, with lot of red lights on the route.

DFWdude

When the main battery is fully charged, ECO Stop/Start reaches working parameters quickly and triggers engine stops regularly. The first time this happens, it reminds me to turn off the "feature" via the button on the center stack. So, I don't use ECO Stop/Start, really.

The other debate... I am NOT convinced that the AUX battery has any role in ECO Stop/Start. It appears to me that the AUX battery does play a role in maintaining electrical continuity for accessories when ECO Stop/Start works, but not to initiate it. Instead, experience confirms for me that ECO Stop/Start IS dependent on whether the MAIN battery is fully charged or not. Lower main battery charge, no ECO... Full charge and it works with a vengeance.

Left Coast Geek

yeah, basically the aux battery maintains all the aux stuff like infotainment/telematics and lighting when the starter is running. old school cars had a load shed relay that shut all that stuff off when the starter is engaged, to save power for starting.

kajtek1

There is also electrical ATF pump, who runs when engine does ECO stop.
I have never seen MB explaining full function of aux battery in those models, but it definitely keeps the lights from dimming.
I guess using ECO or not make the difference on battery life.
I also question "smart battery charging" as not being too smart. I mean on my 1st W212 it was pretty clear, when regenerative braking happened, but now on the 2nd W212 the battery voltage changes without any logical explanation.
Sometimes it is 14.4V, the other time it keeps 12.5V, what is some charging as battery alone would drop below 12V pretty fast.
Those constant charging/discharging surges are what is shortening battery life and we can only remember when MB batteries would last 13 years, or batteries sold with 96 months warranty, like last Bosch I bought about 5 years ago.
Anyway, I enjoy ECO stops during long waits at red lights, when fuel savings are not really measurable.
Meaning I am OK with more frequent battery replacement for the benefits.
Coming to ECO restart, the technology is amazing. It does start in microseconds, using fuel injection on diesels, or plug spark on gassers. Starter seems to be only directional help.
Funny part, I duplicated "ECO" start on my 1922 Ford T.
Doing some work on it, I started/stopped it few times in the garage. Ford has separate ignition switch and separate starter switch.
So at one point I turned the ignition on and Ford engine would start without starter. Evidently the engine stopped with piston just behind top and when I turned the ignition, it got spark..

Left Coast Geek

I think battery quality has gone downhill these past 5-10 years, too.

slovcan

So, what is the consensus here regarding charging the auxiliary battery with a maintainer type charger (float charger)? Disconnect it or not? I got the auxiliary battery message today, although it was quite cold overnight.

Thanks,
Glenn

CaliBenzDriver

You don't need to disconnect anything with a single AUX charger not set for "AGM/Deep Cycle" applications.

> On "AGM settings" a higher voltage is used to equalize cells. That is a risk of zapping 16V capacitors that Mercedes thought would be nice on cars with 14.9V alternators.

> Two chargers connected at once to the same vehicle is not advised... untested common mains issue. Disconnect one of the batteries out as needed.

> As always don't rely on your car supplying normal power above 12.6V. Set uour IC to display voltage while driving. You'll get a chance to see if your Benz is shorting its batteries below 11.xV

slovcan

OK, thanks CBD (hope you don't mind that abbreviation! ). I'll have to dig around to see if I can find how to display the voltage.

Maybe it was just the abnormally cold couple of nights that irritated it and it'll be OK after being on the maintainer for a night.

CaliBenzDriver

Many things can be said in the lead-acid battery chapter. The No1 being these AGM batteries are not built for deep cycles... keep them charged up easily extends life.

Left Coast Geek

FWIW, that may be true for the automotive AGM batts as are used in our Mercedes, but generically, AGM batteries can be very good at deep cycle, they are quite popular in the RV world... but even deep cycle RV lead-acid batteries, AGM or flooded, should be recharged shortly after a deep cycle discharge, and stored fully charged. Those true deep cycle batteries are not appropriate for use as a starter battery as they aren't designed to output the very high currents that a starter motor requires on a cold engine.

kajtek1

Who told you that?
Deep cycle batteries can be AGM, but that is the lowest value for RV use.
Most of the time the best bang for buck are golf cart batteries, or those power hungry spend few thousand dollars on lithium battery bank.
I keep repeating, that the only real advantage of AGM battery in vehicle use, is option of having it upside down.
The option might have value to some, but not to me and I was very sad when Bosch discontinued they flooded batteries.

Left Coast Geek

I've used suitable AGM batteries for deep cycle applicatioons like UPS's,, solar, etc, they do very well at that. yes, flooded lead acid is the best AH per $$, but they need to be externally vented, you need to check the water levels periodically, etc.

Heck, here's Trojan's AGM golf cart and larger true deep cycle batts.... https://www.trojanbattery.com/motive-agm/