S204 C250cdi om651 aux battery

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Tonygw

MB Enthusiast
Joined
May 30, 2019
Messages
1,140
Location
Leicestershire
Car
C250 CDI AMG Blue efficiency diesel
Hi Guys

The stop start eco function has stopped working on my 2012 C250Cdi.
Everything else works fine. So I am suspecting the aux battery has had enough of being a battery.

Firstly, Can I just attach a trickle charger to it, directly to its terminals to see if it will take charge
Secondly Can I just swap it for a like for like replacmeent without any ill affects? I know some cars dont like the battery to be disconnected. Should I disconnect the main battery before I attempt to replace the aux one?

Thanks
 
Yes you can connect a charger direct to the battery, though I would recommend using a modern battery conditioner such as a CTEK rather than an old fashioned battery charger.
Yes you can replace the battery with like for like with no problems & if you decide to replace the battery you do not need to disconnect the main battery just make sure the key is out of the ignition.
 
Great cheers for the reply. I have a modern battery conditioner that I got from Aldi. It switches between car and bike batteries but also can be left on to keep a battery in good condition if left for any period of time. Hopefully it will be ok, it kept the main battery topped up during the worst of the lock down. Cheers
 
@Tonygw Where is the aux battery in the s204? I have a similar problem with my start/stop.
How do you charge it, i presume this is not the same battery as the main one under the bonnet?
 
Ah, it's in the boot under the boot floor in a plastic housing on the right hand ,drivers side. Next to the spare wheel. You have to remove some fasteners to get the plastic housing off. Cheers
 
Ah, it's in the boot under the boot floor in a plastic housing on the right hand ,drivers side. Next to the spare wheel. You have to remove some fasteners to get the plastic housing off. Cheers
@Tonygw thanks for that, i presume you just attach the battery charger to the battery terminals insitu to charge it?
 
Yeah I believe so I was hoping for somebody to confirm that's ok on here and somebody did earlier so that's my plan. Cheers
 
Have you had the fault codes read? An one were found?

Please keep in mind however that it could also be the main battery, not the accessory battery, that is at fault.

The main battery might be fine for everything apart from supporting the Stop/Start function... unfortunately in the absence of relevant fault codes there's no way of knowing, other than by replacing the batteries one at the time.

Other common causes are failed Brake Vacuum Sensor at the front of the brake servo, or failed Battery Management Module on top of the positive battery terminal, but these are likely to leave a fault code.
 
Cheers for the reply. No fault codes are stored so I am guessing it could be the battery, but like you I don't like guessing.
 
This is where it lives - on the right-hand side.

cccpng.png
 
This is excellent news.. Why do you want the annoying stop start working ?

Battery management modual is on the negative terminal not positive , if that was at fault brake hold would also not work .
 
Thanks for the picture MarkJay. I did charge the main battery but no difference, no fault codes either, so my next logical step in my mind was to charge the aux battery and see what happens. I know for the stop start to work there are many parameters that need to be met but its definately not meeting one at least and I suspect the battery. Although every other elctrical function is fine on the car.

Gazwould, I hate the stop start system LOL but I also hate it when things dont work, more!!.. So I just want to get it right and then I will still turn it off every drive.. But the fact its not working is bugging me like crazy..
 
OK get it fixed , not sure why as probably £ for a new aux .

I've pulled the 7.5A fuse to the auxiliary battery and it's stop/strain disabled bliss .
 
Does the brake hold function or any other functinos get affected by pulling the fuse?
 
Thanks for the picture MarkJay. I did charge the main battery but no difference, no fault codes either, so my next logical step in my mind was to charge the aux battery and see what happens. I know for the stop start to work there are many parameters that need to be met but its definately not meeting one at least and I suspect the battery. Although every other elctrical function is fine on the car.

Gazwould, I hate the stop start system LOL but I also hate it when things dont work, more!!.. So I just want to get it right and then I will still turn it off every drive.. But the fact its not working is bugging me like crazy..

I know what you mean... I do switch it off when I start the car, and refer to the ECO button as the 'battery condition check button' - every few days I turn it on for a brief moment while driving just to see the reassuring green ECO come up on the instrument cluster :thumb:

Also, I do use the HOLD function regularly.
 
OP. Have you had the software update that came out of dieselgate? My S212 had it and the Eco system hasn’t worked as regularly as before since the update. (It’s worked once since the update on Nov) I took the aux battery out to charge but it made no difference. It’s not a function I’m too bothered about so won’t take it any further.
 
HI, No never had an update. I dont use main dealers so proably never will get it. I will charge the aux battery though see what happens.
 
@Tonygw I have just put my aux battery on charge, see what happens.
 

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