W211 Dead Starter Battery?

[E55BOF]

I'm sick of the Coronavirus saga, so now for something completely different...

A couple of days ago I tried to start the 2003 W211, but all I got was the 'Convenience Functions Disabled' message. The engine did not turn over, nor even attempt to.

My understanding of that there electrickery is not the best, but as I understand it, even if the auxiliary battery is low the starter battery should have ample power to start the engine.

Is what happened telling me that the starter battery is dead, or can it happen even if that's not the case?

 

[pmcgsmurf]

The understanding of the battery system in the W211 can be a bit confusing.
There are 2 batteries in the early 211's, one in the boot and one in the engine bay, on rhd below the pollen filter.

The one in the engine bay (aux) is in case the main battery fails so that the SBS system can still be powered.
There is a battery control module that monitors things and switches charge to the aux battery to keep both topped up.

If the car is not starting try and charge the main battery, this is the one that starts the car.
I keep a ctek on the W211 and this keeps both batteries topped up as if the aux battery goes low the control module switches over to this.

 

[Azeem]

May I ask how do you charge the main battery in boot, without disconnecting the battery terminals?
Thanks

 

[E55BOF]

I've charged the battery (Ctek again) using the underbonnet charging points, and the car started fine. I understood, though, and others have said the same, that the battery in the engine bay is there only to provide power to start the engine, and the main battery in the boot does everything else. That was the setup on my R230, of similar vintage, which also had the SBC. The internet offers both interpretations...

PmcGSmurf, I don't say you're wrong, but how sure are you?

And I'm wondering now - does the estate have the main battery in the boot/load area, or under the rear seat on the RH side? I'm sure one of my Mercedes had it there, but that may have been the W210. I've had far too many cars...

May I ask how do you charge the main battery in boot, without disconnecting the battery terminals? Thanks

If you have an estate, you have charging points under the bonnet. If you have a saloon, I believe that you don't have the charging points, and presumably need to remove the boot floor trim to get at the battery.

 

[pmcgsmurf]

May I ask how do you charge the main battery in boot, without disconnecting the battery terminals?

You can do either, for a completely dead battery I'd remove the terminals, in fact I'd take the battery out and leave it in the garage on a charger for 24 hours as the battery if completely dead could draw as mush as 20 amps from the charger.

For just topping up, keeping the battery full attaching eyelets on the battery terminals with a bayonet connector of some style works ok.

 

[AMGeed]

I've charged the battery (Ctek again) using the underbonnet charging points, and the car started fine. I understood, though, and others have said the same, that the battery in the engine bay is there only to provide power to start the engine, and the main battery in the boot does everything else. That was the setup on my R230, of similar vintage, which also had the SBC. The internet offers both interpretations...

PmcGSmurf, I don't say you're wrong, but how sure are you?

Pat is correct. What you describe above only applied to the R230.

 

[E55BOF]

Thank you. It's a bit daft, though; if the R230 needs a backup to run the SBC in case of battery failure , why doesn't the W211? I suppose it would get silly, because if the W211 needs two, then to cover that contingency as well the R230 would need three... I can't help wondering why they're not the same.

 

[pmcgsmurf]

I've charged the battery (Ctek again) using the underbonnet charging points, and the car started fine. I understood, though, and others have said the same, that the battery in the engine bay is there only to provide power to start the engine, and the main battery in the boot does everything else. That was the setup on my R230, of similar vintage, which also had the SBC. The internet offers both interpretations...

PmcGSmurf, I don't say you're wrong, but how sure are you?

I'm pretty sure, I was wrong once but that was back in 1974.

I remember reading about the "starter battery" in the engine bay and my first thought was that the battery there is way to small too supply the current required to the starter to turn the engine over.
Sure I had a document somewhere that explained the system as I had a problem a few years ago with a Battery Control Module so needed to investigate and find out what was wrong in the car, let me see if I can find it again.....


Found it, it's too big to add the PDF document here.
The relay K57/2 is not energised until the engine is running so the small battery cannot supply the starter.

And I'm wondering now - does the estate have the main battery in the boot/load area, or under the rear seat on the RH side? I'm sure one of my Mercedes had it there, but that may have been the W210. I've had far too many cars...

Pretty sure the battery on the S211 is still in the back on the right side.

 

[Chrishazle]

CTEK is a conditioner not a full blown charger, as there was only 2 hours between your post 1 and reporting getting the car started, a conditioner would not have put very much charge back into the battery, so I'd suggest either putting the CTEK onto the main battery and leaving it there until you next need the car - at least 48 hours, maybe more. If you can't be without the car for that long, I'd suggest you get a 6-8amp intelligent battery charger.

I have an S204 rather than s211, only 1 battery (in the boot under the floor OS) but it is 100AmpHours (AH) so from showing half charged a few weeks ago it would need 50AH to get back to full charge - over 6 hours on an 8 amp charger and a lot longer on a conditioner.

 

[mersum1es]

I also think ctek won't charge aux battery, because relay is engaged for charging only right after start if BCM founds low voltage in aux. Car in sleep won't drive relay.

 

[E55BOF]

I charged the battery two days ago. I put an old 10-amp charger on it initially, and it started charging at 8 amps, dropping to about 6 amps after a couple of hours. The car started then, and I took it for a run to get some stuff from the supermarket - about twenty miles in all (I took the longer, but faster, route via the motorway). I put the Ctek on it after I got back, and it was fully-charged in the morning.

The battery may be getting tired, but that's not a problem; a new one is cheap enough. The only thing I don't understand now, is why when I first tried to start it there was no attempt to turn the engine over, no slow whur-whur-whur; not even a clunk, but no dimming of the dashboard warning lights. Does the system sense when there's not enough in the battery, and make no attempt to energise the starter circuit?

 

[pmcgsmurf]

Does the system sense when there's not enough in the battery, and make no attempt to energise the starter circuit?

Yes, you have to not think of these cars as old cars where you turn the ignition on then when you turn the key yo start all you are doing is energising the solenoid on the starter.

On these cars you don't really start the car, you just tell it to start then the electronics take over.

 

[Major774]

It should still attempt to start with some juice. It should at lease engage the rod which the pinion gear is attached to, so you will get metal clunk sound.

 

[E55BOF]

I didn't, though, but see Post #12 for why, if I understood it correctly.

 

[Major774]

I get what Patrick is saying, but as you have met all the starting requirements (i.e. immobiliser, gear selector, etc.) it should have at least attempted to start.

Unless you have an intermittent electrical fault?

 

[DSM10000]

I get what Patrick is saying, but as you have met all the starting requirements (i.e. immobiliser, gear selector, etc.) it should have at least attempted to start.


Unless you have an intermittent electrical fault?

If the monitored battery voltage is below a certain threshold then power is not applied to the starter circuit.

 

[Chrishazle]

If the monitored battery voltage is below a certain threshold then power is not applied to the starter circuit.

That's exactly what happened to me about 3 weeks ago with the S204 - opened car, dash lit up, key in and turned, nothing happened. Battery condition tester showing battery only 50% charged. Put charger on it about noon, initially pulling 10 amps but quite quickly dropped to 5, left it until about 6pm then swopped charger for conditioner and left it overnight, next morning all seemed to be back to normal - but within a week of little or no use, could not even open the car, had to use the emergency key to open the drivers door then bonnet, put charger on emergency +ve and bracket on side of engine, continuous rapid clicking noises under bonnet and dashboard lights flashing so turned it off and called our breakdown company, he somehow got enough juice in to open the boot so we could get at the battery, eventually jump started the car, battery was goosed (4 months old only!) and had to be replaced.

 

[E55BOF]

When you say 'battery condition tester', I presume you mean something like this?

6V 12V Digital Battery Load & Charging System Tester Car/Van Battery Test 717493113352 | eBay

They all appear to be much the same, though I wouldn't buy that one on principle - it's described as 'digital', and it has digits on it, but that's not the accepted meaning of 'digital'...