S204 C250cdi om651 aux battery

[Tonygw]

Ah cool let me know, did you leave it connected in the car?

 

[mpc]

Ah cool let me know, did you leave it connected in the car?

@Tonygw yes, i have left it connected in the car and just attached the battery charger to it.

 

[Gazwould]

AGM setting ?

 

[Tonygw]

Fingers crossed it's all good. Might put mine on if I get chance today.

 

[markjay]

If the car is in regular use, and yet the Stop/Start isn't working due to either of the two batteries not having sufficient charge, then I doubt that charging the batteries with a trickle charger will resolve the issue. That said, for what it costs, it must be worth a shot.

But ultimately you'll need to take a punt on which battery you replace first... the auxiliary battery is cheaper (if opting for OEM and not genuine), but the main battery works harder and more likely to be the one that failed - especially if you regularly switch-off the Stop/Start function.

You might want to invest in one these, it could just give you the information you need:

Battery Drop Tester 6/12V | BT91/7 | 1 Year Guarantee | Sealey

American-style drop tester. Load applied by internal switchgear - no sparking! Suitable for 6 and 12V batteries. Applies load across cells and measures output on meter.

www.sealey.co.uk

Around £30 from eBay.

 

[markjay]

The input from the battery managment module is a prerequisite for Start/Stop to function, so if you permanently disable it then you will in effect also permanently disable the Stop/Start function.

Disabling the battery managment module will have some effect on battery life and fuel consumption, but whether it will actually be significant - or even noticeable - is debatable.

See also:

Battery sensor: how it works, problems, checking, battery replacement

What is a battery sensor in a car, problems, checking, replacement; changing a battery with a battery sensor

www.samarins.com

 

[mpc]

AGM setting ?

Not for the AUX battery no...

 

[Tonygw]

Just an update on this, I still ahvent resolved the issue. However, When I tested the aux battery (engine off!) it was reading 13.9-14.2V Which it thought was a little odd. The main car battery was around 12.4V.
I would expect the battery to go up to say 14V while the engine was running but never while connected to the car engine off.
To me the main batery seems fine but the aux battery seems to be an odd voltage at nearly 14V. I connected the smart charger to it and it immediately said the battery was 100% charged.

I havent load tested the battery yet, thats next thing to do. My thought is the battery may have failed and is simply ready high battery volts but unable to deliver any current?
I dont have much experinece with AGM batteries, but I have had lithium batteries fail in this manner before. High volts no current.

Does this sound like that battery has failed?

Thanks

 

[markjay]

Had a look on Google, apparently a battery voltage higher than 12.9V is an indication of a dying battery.

(It is referring to main battery, but same logic applies)

 

[Tonygw]

Great thanks, Right, so I have ordered an aftermarket AGM battery of the same specification as the original fitted.

Next question, what is the correct procedure to change the aux battery? Can I just disconnect it and connect the new one? Or do I need to disconnect the main battery first?

Either way, which is the correct terminal to disconnect first is it negative first? or posotive first?

These newer cars are way more sensitive than the older ones I am used to.

Thanks

 

[markjay]

From memory, the auxiliary battery on my W204 is traditional wet Lead-Acid battery, not AGM, made by FIAMM.

 

[Tonygw]

What year was your car? Mines 2012 but its definately AGM. But I will check again to be 100% sure...

Any idea on the correct precedure to change it?
Cheers

 

[GeorgeR]

I have done some investigation on the AUX battery. It looks the car checks its status after every start and if the voltage is not healthy enough then it displays the AUX battery warning alert and turns off all AUX battery-related features: HOLD, Start-Stop, Autopark, etc. The required idle (unloaded voltage) should be above 12.85 ~ 12.90 V in my car (with AGM batteries). The charger / voltage control logic is quite smart: after each startup, it sends a test burst (14.8V, 1 sec) to the battery and reads the reaction (Note that it adds voltage only if the car has started moving (5 mph or so) and keeps it just a little longer than the car slows down.) It charges with increasing voltage up to 14.90 V till the idle voltage reaches the 13.20 Then it switches to trickle charging to keep the battery between 13.20 - 13.50 V while the car runs. This means it does everything to keep this battery fully charged. On the other hand, if your battery cannot be at least 90% charged (idle unloaded is not above 12.9V) it reports failure. For me, it came after a good 6 years so it was time to change the battery even it looked quite OK for many other use cases.

As for the start stop: luckily my E class Automatic one has the E- S modes button where the S disables the Start Stop simply. Is it not there in the shifters or in other models?

 

[Gazwould]

The brake HOLD function is nothing to do with the aux battery , HOLD works even with aux battery 7.5A fuse pulled which disables the SS abomination.

The whole S/S thing amazes me , quite simply bad.. and then people take a guess on which battery .

By all means if OCD dictates get it working and then disable by pulling the fuse knowing it can be reactivated if moving on .

Mines been disabled for the most part of the year now and when I've gone to charge the aux a couple of times since it's already up there and takes just a few minutes for the Maypole to give the thumbs up.

 

[GeorgeR]

There might be quite a few different logical setups though. In my case (yr 2014 s212.298 / Hybrid) I am absolutely positive, that the two things are strictly interconnected logically.
Whenever the battery does not have the sufficient voltage I have yellow READY on the main dash there is no hold (and no autopark and no start stop). (As opposed to the start stop, the HOLD does not need to meet any other conditions: outside temp, engine temp, etc, just this one.) I have measured it on the fly dynamically. Whenever the voltage of the AUX battery is fine after a restart the comfort features are on with the HOLD.

 

[Gazwould]

Yes there are variations but the thread is about a 2012 S204 , going to a different model could be misinformation .

As I've the same car as OP , my best advice is pull the damn fuse , lol .

People are falsely being made to believe it is a good thing whilst it is not .

 

[Fez7]

I know I'm reviving an old thread here but I wonder if the OP or anyone with similar issues got this resolved through charging the aux battery.

My stop start has not worked since I bought the car in April. Like most I'm not bothered about S/S but it does bug me that the car isn't as it should be. More recently the brake hold function has stopped working completely, a couple of months ago it stopped working intermittently but now it's permanent.

The car was with a specialist yesterday who confirmed there are no fault codes showing. I bought a Maypole smart charger the other day and used it on the main battery. When I checked on it after a couple of hours it was already green, it could well have been much sooner but I had anticipated having to leave it for much longer hence why I didn't check it sooner.

I have just used it on the aux battery and within only a couple of minutes it has turned green on 13.6v. I took the car for a very short test drive but still no S/S or brake hold working.

Not sure what to do next. Should I leave the charger on both batteries overnight first just in case? Or should I just bite the bullet and replace the batteries starting with the auxiliary first since it's cheaper? If so, should I only go for an OE battery from the dealership or just get an aftermarket? I'm planning on keeping the car for 3 years so want it to last. It has full MB service history so every other part on the car is genuine MB rather than aftermarket.

 

[alecmascot]

I replaced the main battery to fix this issue.
Varta's are fine, but MB may be competitive on price.

 

[GeorgeR]

I know I'm reviving an old thread here but I wonder if the OP or anyone with similar issues got this resolved through charging the aux battery.

My stop start has not worked since I bought the car in April. Like most I'm not bothered about S/S but it does bug me that the car isn't as it should be. More recently the brake hold function has stopped working completely, a couple of months ago it stopped working intermittently but now it's permanent.

The car was with a specialist yesterday who confirmed there are no fault codes showing. I bought a Maypole smart charger the other day and used it on the main battery. When I checked on it after a couple of hours it was already green, it could well have been much sooner but I had anticipated having to leave it for much longer hence why I didn't check it sooner.

I have just used it on the aux battery and within only a couple of minutes it has turned green on 13.6v. I took the car for a very short test drive but still no S/S or brake hold working.

Not sure what to do next. Should I leave the charger on both batteries overnight first just in case? Or should I just bite the bullet and replace the batteries starting with the auxiliary first since it's cheaper? If so, should I only go for an OE battery from the dealership or just get an aftermarket? I'm planning on keeping the car for 3 years so want it to last. It has full MB service history so every other part on the car is genuine MB rather than aftermarket.

I would try the test / change of AUX battery first for sure. The change should not cost more than 70 GBP (if you buy the OEM Bosch).
For testing you will need to measure it while the car drives. (See the readings in my old post in this thread.)

 

[Gazwould]

I know I'm reviving an old thread here but I wonder if the OP or anyone with similar issues got this resolved through charging the aux battery.

My stop start has not worked since I bought the car in April. Like most I'm not bothered about S/S but it does bug me that the car isn't as it should be. More recently the brake hold function has stopped working completely, a couple of months ago it stopped working intermittently but now it's permanent.

The car was with a specialist yesterday who confirmed there are no fault codes showing. I bought a Maypole smart charger the other day and used it on the main battery. When I checked on it after a couple of hours it was already green, it could well have been much sooner but I had anticipated having to leave it for much longer hence why I didn't check it sooner.

I have just used it on the aux battery and within only a couple of minutes it has turned green on 13.6v. I took the car for a very short test drive but still no S/S or brake hold working.

Not sure what to do next. Should I leave the charger on both batteries overnight first just in case? Or should I just bite the bullet and replace the batteries starting with the auxiliary first since it's cheaper? If so, should I only go for an OE battery from the dealership or just get an aftermarket? I'm planning on keeping the car for 3 years so want it to last. It has full MB service history so every other part on the car is genuine MB rather than aftermarket.

Aux battery has nothing to do with brake HOLD.