C-Class 220 CDi (W205) (2015) - Auxiliary battery problem

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This question relates to the above C-Class, just under 4 years old, with only 23,000 miles on the clock.

The "Auxiliary Battery Malfunction" warning flashed up on the instrument cluster a couple of days ago. Although the warning isn't critical - and the car is perfectly driveable - the warning continued to flash every few seconds. Remember when malfunctions alerts simply came up and could then be stored in the malfunction memory without being a pest?

Anyway... the next day I took the car for a run of about 50 miles or so and the warning disappeared.

I've being trying to figure out - so far without success - whether the problem could have arisen because the car isn't often used for anything other than short local journeys; and it really doesn't need replaced.

I have an approved used warranty expiring next month that would cover a replacement. I'll probably take out a Tier 1 warranty after that, but I'm not sure it would provide cover for this. That policy has a lot of wriggle room in the area of "fair wear and tear".

So, the question is, at this low mileage, is it just possible there's no fault and the aux battery just needs a decent charge from time to time?

Incidentally, I have a feeling that the aux battery kicks in when the car stops in Eco Stop/Start mode, but I disable that at the start of every journey. Apart from being a pest, it hardly ever works. In fact, it will only work when the weather is blistering hot; and where I live that doesn't happen very often.
 

Gallon

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The (aux batt malfunction) warning is misleading, as your car doesn't have an aux battery, it has a large capacitor which Mercedes call a Voltage Converter. This is located in the passenger footwell below the glove box. They are known to fail & yours could be on it's way out.
 

markjay

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The reason that your Stop/Start does not work is that your auxiliary battery (or capacitor, as above) is faulty.

The auxiliary battery provides power to the electrical consumers when the engine is not turning. This arrangement keeps the main battery free for starting the engine. Without the auxiliary battery, there's no power for electrical consumers and the engine won't switch off (i.e. Stop/Start won't work).

Once you have replaced the auxiliary battery, the Stop/Start functionality will work normally again (though you can obviously still switch it off if your don't like it).
 
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Thanks to Gallon and markjay for the helpful replies.

Although the problem seems to have gone away for the time being, I'll try to get it replaced under warranty at a service next month.

When the malfunction light was appearing, the fault didn't appear to have any effect on the availability of electrical consumers with the ignition off. The stop/start didn't work, but as I said in the original post it rarely does. I acquired the car about 10 months ago at around 19,000 miles. The function worked during hot summer weather and then packed up in the autumn. It worked briefly a couple of weeks ago on a very hot day. I know that a number of conditions need to be satisfied before it will kick-in: e.g., sufficient oil temperature. But I've also heard that it can sensitive to the ambient temperature.
 

markjay

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Thanks to Gallon and markjay for the helpful replies.

Although the problem seems to have gone away for the time being, I'll try to get it replaced under warranty at a service next month.

When the malfunction light was appearing, the fault didn't appear to have any effect on the availability of electrical consumers with the ignition off. The stop/start didn't work, but as I said in the original post it rarely does. I acquired the car about 10 months ago at around 19,000 miles. The function worked during hot summer weather and then packed up in the autumn. It worked briefly a couple of weeks ago on a very hot day. I know that a number of conditions need to be satisfied before it will kick-in: e.g., sufficient oil temperature. But I've also heard that it can sensitive to the ambient temperature.

When the Stop/Start isn't operational, the main battery works as per normal i.e. it supplies both the starter motor and the other consumers. In fact there's no need to replace a faulty auxiliary battery if you don't care about the Stop/Start (but obviously if it's under warranty then it would not be sensible not to do so).

And although you are correct that there are a number of conditions required for the Stop/Start to work, 9 out of 10 it will be either a fulaty main battery or a faulty auxiliary battery - if either is not 100%, the Stop/Start won't work.
 
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Frankly, were it not for the infernal "nag" message about the malfunction in the instrument cluster - which IMO ought to be capable of being cancelled and stored in the case of a non-critical fault - in the light of what you say but for the warranty I wouldn't bother with a replacement.
 

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Ours was replaced under warranty just over a year ago.
 
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... at the time of the repair, I should have said.
 

markjay

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Google:

"Auxiliary Battery Failure" Mercedes

Early failures (within the warranty period) are very common.
 

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Bobclive2

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My C Class Coupe is Dec 2011 and has both original batteries, I don`t use stop/start. The reason your batteries are not being fully charged is due to the smart charging system where the engine management unit controls the alternator and won`t allow it to charge the battery from engine power when SOC is over 75%, this is to allow room for any regeneration, no regeneration no charge = near to flat battery if car is little used or does mainly short journeys , to test this just disconnect the sensor attached to the negative rail of the starter battery, drive the car for a few days, a few long runs would be good and the battery should then be near fully charged and remain so, you will loose stop/start and brake hold while the sensor is disconnected, there will be NO eml and your battery warning light should go out. Visit this link on the SLK forum, read to the end.

Start / Stop / eco tech a load of Bull ...... - Mercedes Benz SLK Forum
 
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Bobclive2

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Quote "Auxiliary Battery Failure" Mercedes

Early failures (within the warranty period) are very common. Quote.

Only if stop/start is implemented, disconnect the sensor get rid of stop/start and smart charging and Mercedes batteries will last as long as mine, by the way, my car also does mainly short journeys and is used as a second car.
 

markjay

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My 2013 W204 has both original batteries, and stop/start still works fine - I guess I'm lucky.
 

UrbanM

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The reason that your Stop/Start does not work is that your auxiliary battery (or capacitor, as above) is faulty.

The auxiliary battery provides power to the electrical consumers when the engine is not turning. This arrangement keeps the main battery free for starting the engine. Without the auxiliary battery, there's no power for electrical consumers and the engine won't switch off (i.e. Stop/Start won't work).

Once you have replaced the auxiliary battery, the Stop/Start functionality will work normally again (though you can obviously still switch it off if your don't like it).
That’s so useful. I’m having The same issues.
 
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Update my end: The auxiliary battery was replaced and since then the stop/start function hasn't worked once... the amber icon stays on all the time. In the two years I've had the car, the feature has only worked on a handful of occasions, usually on a long run in hot weather.
 

UrbanM

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Update my end: The auxiliary battery was replaced and since then the stop/start function hasn't worked once... the amber icon stays on all the time. In the two years I've had the car, the feature has only worked on a handful of occasions, usually on a long run in hot weather.
Thanks for the update. Much appreciated.
 

RTR1879

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Mine is doing the same thing. Start/stop has worked very rarely since I got the car in November (67 plate C220d AMG Line), probably 2 or 3 times in total and not at all since probably mid-December. I think I'll give the dealers a call tomorrow and get it looked at under warranty.
 

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Mine is doing the same thing. Start/stop has worked very rarely since I got the car in November (67 plate C220d AMG Line), probably 2 or 3 times in total and not at all since probably mid-December. I think I'll give the dealers a call tomorrow and get it looked at under warranty.
Try giving your main batteries an overnight charge with a reliable charger and see if your star/stop will work. Chances are your main battery does not have sufficient charge to satisfy the start/stop condition.
 

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