W205 C200 showing "Auxiliary Battery Malfunction"
Discussion
Hi all,
My 2017 Mercedes C200 (W205, around 74k miles) has recently started showing an "Auxiliary Battery Malfunction" message every time I start the car.
The warning only stays on for a few seconds and then disappears again. Apart from that the car seems completely normal to drive. No other warning lights, no rough running, nothing obvious really.
The only thing I have noticed is that the stop/start system doesn't seem to work anymore.
I haven't taken it anywhere yet as the car otherwise feels absolutely fine, but after a bit of searching online it seems like this might be a fairly common W205 issue?
Just wondering if anyone here has had the same thing and whether it turned out to be something simple or more expensive in the end.
Cheers
My 2017 Mercedes C200 (W205, around 74k miles) has recently started showing an "Auxiliary Battery Malfunction" message every time I start the car.
The warning only stays on for a few seconds and then disappears again. Apart from that the car seems completely normal to drive. No other warning lights, no rough running, nothing obvious really.
The only thing I have noticed is that the stop/start system doesn't seem to work anymore.
I haven't taken it anywhere yet as the car otherwise feels absolutely fine, but after a bit of searching online it seems like this might be a fairly common W205 issue?
Just wondering if anyone here has had the same thing and whether it turned out to be something simple or more expensive in the end.
Cheers
Stop/Start cars have two batteries, the main starter battery which is normally in the engine bay and an auxiliary battery to support 12v systems when the engine is not running.
On the W205, that battery is in the boot, sounds like yours needs replacing.
ETA: Actually, I am now doubting myself... I've had a W204 C200 diesel and a W205 C200 petrol hybrid... One definitely had an auxiliary battery in the boot... but which one I am not sure...
On the W205, that battery is in the boot, sounds like yours needs replacing.
ETA: Actually, I am now doubting myself... I've had a W204 C200 diesel and a W205 C200 petrol hybrid... One definitely had an auxiliary battery in the boot... but which one I am not sure...
Edited by Megaflow on Thursday 28th May 10:20
In the coupe it’s in the passenger footwell area behind the carpet.
Possibly the same on the saloon. It’s fairly straightforward to get to, probably DIYable
I believe it is only used for stop-start as opposed to anything else. It’s not a battery in the conventional sense but a capacitor.
Possibly the same on the saloon. It’s fairly straightforward to get to, probably DIYable
I believe it is only used for stop-start as opposed to anything else. It’s not a battery in the conventional sense but a capacitor.
OK, let's get down to business.
Bizarrely, the W205 doesn't actually have a second battery at all. Yes, I know that's what the warning says, but it's lying. There is no second battery in any recognisable sense of that word.
The thing under the carpet in the passenger footwell is some sort of capacitor device that's used for the start/stop system. And, guess what, they fail very frequently. So much so that your friendly Merc dealership will sell you a reconditioned one on an exchange basis for not very much cash at all.
Ask me how I know....
Bizarrely, the W205 doesn't actually have a second battery at all. Yes, I know that's what the warning says, but it's lying. There is no second battery in any recognisable sense of that word.
The thing under the carpet in the passenger footwell is some sort of capacitor device that's used for the start/stop system. And, guess what, they fail very frequently. So much so that your friendly Merc dealership will sell you a reconditioned one on an exchange basis for not very much cash at all.
Ask me how I know....
The stop/start failing to work is a sign the car isn't getting used enough, or you need to put the car on a battery conditioner periodically - I replaced both batteries on my CLS last year and due to lack of use the stop/start doesn't work unless the car has had plenty of use or I give it a quick charge up. Can't comment on the aux. battery on a 205 but they aren't expensive generally if you did need to replace it (assuming it has one!)
W205 is a curious creature, not as simple as it looks. As mentioned above there is no second battery, just some sort of capacitor.
One feature of W205 is the intelligent alternator which is clutched in and out of use as required.
Energy Recuperation: When you lift your foot off the accelerator or press the brake pedal, the alternator is engaged and the car s kinetic energy is converted into electrical energy.
Alternator Disengagement: To save fuel, the alternator clutch disengages and stops charging while you are accelerating. It only reconnects under heavy loads or when the battery requires charging.
Smart Charging: The smart charging module regulates the alternator output so that most of the charging happens during deceleration.
I have a strong suspicion that when a W205 is driven gently with very little use of the brakes the star/stop capacitor may never get charged at all. This can give the impression that the system isn't working or that the main battery is weak when, actually, everything is fine.
One feature of W205 is the intelligent alternator which is clutched in and out of use as required.
Energy Recuperation: When you lift your foot off the accelerator or press the brake pedal, the alternator is engaged and the car s kinetic energy is converted into electrical energy.
Alternator Disengagement: To save fuel, the alternator clutch disengages and stops charging while you are accelerating. It only reconnects under heavy loads or when the battery requires charging.
Smart Charging: The smart charging module regulates the alternator output so that most of the charging happens during deceleration.
I have a strong suspicion that when a W205 is driven gently with very little use of the brakes the star/stop capacitor may never get charged at all. This can give the impression that the system isn't working or that the main battery is weak when, actually, everything is fine.
Megaflow said:
ETA: Actually, I am now doubting myself... I've had a W204 C200 diesel and a W205 C200 petrol hybrid... One definitely had an auxiliary battery in the boot... but which one I am not sure...
W204 and S204 C200 diesels - the little battery is in the boot. Mine is knackered which is great as it means the stop/start doesn’t work. No warnings or Anything, just no pain in the arse stop/start. Dog Star said:
Mine is knackered which is great as it means the stop/start doesn t work. No warnings or Anything, just no pain in the arse stop/start.
I like your style. Stop/start is IMO a gift from hell. In one of my cars I've had it programmed out. In the the other (W205) I turn it off as soon as I get in the driver's seat. Hence the accidental knowledge about W205s...Yes, W205s charge the battery and stop/start system when you lift off the throttle and/or hit the brakes. Took me a long time to get to the bottom of it.
It might not be "regenerative braking" but it's definitely "regenerative charging".
Panamax said:
W205 is a curious creature, not as simple as it looks. As mentioned above there is no second battery, just some sort of capacitor.
One feature of W205 is the intelligent alternator which is clutched in and out of use as required.
There's nothing unique about that - Intelligent alternators / smart charging have been common for years. Wife's old mk1 Tiguan had it - grouped under the BlueMotion features. I think such things are on all cars now.One feature of W205 is the intelligent alternator which is clutched in and out of use as required.
Dog Star said:
Megaflow said:
ETA: Actually, I am now doubting myself... I've had a W204 C200 diesel and a W205 C200 petrol hybrid... One definitely had an auxiliary battery in the boot... but which one I am not sure...
W204 and S204 C200 diesels - the little battery is in the boot. Mine is knackered which is great as it means the stop/start doesn t work. No warnings or Anything, just no pain in the arse stop/start. The capacitor thing is interesting though, I have always though twin batteries for stop/start is a bit of an over kill.
Thanks for all the replies so far.
I finally got around to plugging the car in and this is the fault code that came back.
B210D00
Reading back through the thread, it sounds like quite a few of you were on the right track with the capacitor/voltage converter unit in the passenger footwell.
The car still starts and drives perfectly fine. The only things I've noticed are the warning message at startup and the stop/start no longer working.
Am I basically looking at replacing that footwell unit then, or is there anything else worth checking first?
I finally got around to plugging the car in and this is the fault code that came back.
B210D00
Reading back through the thread, it sounds like quite a few of you were on the right track with the capacitor/voltage converter unit in the passenger footwell.
The car still starts and drives perfectly fine. The only things I've noticed are the warning message at startup and the stop/start no longer working.
Am I basically looking at replacing that footwell unit then, or is there anything else worth checking first?
An aftermarket replacement is available on Amazon from about £22 which sounds an attractive possibility.
https://www.amazon.co.uk/Orivin-Converter-20544000...
https://www.amazon.co.uk/Orivin-Converter-20544000...
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