Talk Morgan Forums Community The Malvern Arms Talking of batteries….

This is about a diesel Merc. I have driven it for about 4 years and the auto-stop feature has only worked (I think) twice, each time after a very long drive.
I presumed that this was because of standing idle every other day.

I’ve had a month off driving, with a small AA solar cell trickling a charge in. Today on returning to the car I got the warning message “please start engine, battery low”. I drove about 20 miles and the auto-stop started working. I wonder what’s going on. No thread drift please.

Most modern Mercedes have an auxiliary battery which last about 5 to 7 years, you start to notice the aux battery is on the way out when functions like stop start, pre tensioning of the seat belts either stop working or become intermittent and eventually you will get an aux battery warning.

At risk of drift, I had a Mazda 3, 2.2litre diesel and stop-start would only function on that (by design) if the engine was hot enough, so on short journeys it would rarely operate. Could be worth perusing the operating manual to see if Mercs are designed similarly.

The Mercedes stop start is some times subject to what gearbox mode you use eg economy, in the applicable modes the stop start will always be on and you have to push a button to deselect it, mine stopped working and it required replacement of the aux battery and the stop start circuit board replaced. Many ECU remaps will deactivate it permanently.

We had something similar with our F54 Mini. We bought it at one year old and stop/start feature worked regularly. After a few years it started to auto-inhibit with a corresponding brief message. Later still the AGM stop/start battery was replaced and after warming up it now regularly works again - rather annoyingly on most occasions! It can easily be overriden but resets on next startup

In your case it's highly likely the battery's capacity has diminished with normal ageing, to such a level that it can be re-charged to full remaining capacity quite quickly. The monitoring software see's a fully charged battery and allows stop/start function, whereas previously it would have required a longer journey. With such a limited capacity no amount of static charging will improve the situation significantly I'm afraid. The battery low message further confirms it's time for a new one.

After replacement It's very likely you will need to have the adaptive charge profile reset in the cars ECU to match as new. A message may appear asking for such, although car can be driven. Any garage should be able to perform this in a matter of seconds after plugging their tablet device into OBDII port and choosing your model.

Don't forget to ensure you get an AGM battery. I use a solar cell battery charger plugged into the 12v/Cig lighter to keep the good ladies F56 on charge. It is meant to have stop-start but it has never worked since brand new. It is an Auto which the rep said has never worked with S/start.

Not a good idea on your Mini Alistair. The power socket is controlled by BCU and times out a few minutes after stop so your battery conditioner won't be doing much. Not a good idea anyway TBH. Under bonnet the positive battery terminal has an extension and cover for easy access and be careful to use the provided negative extension as well. The seemingly convenient top suspension mount bolts don't provide a battery return. I did wonder if it's an isolated return system as on a boat.

I agree about replacing with correct AGM battery though. On our F54 Cooper D the surprisingly large 80 Ah battery must be the same physical dimensions as OE because the slide in tray under air cleaner gubbins has no provision for securing clamp to accomodate other than full length battery. Don't ask how I know this

From what you say, it seems the auxiliary battery doesn’t get charged enough on my journeys but the main battery does, however the aux has been charged up by the solar cell via OBD socket while the car has been idle!

I not too sure what reliance may be placed on forum comments but I have seen a few postings on modern Mercedes diesels which suggest that using the stop start for short/commuter journeys may lead to a quicker build up of soot particles in the DPF (diesel particle filter). which may if not regularly regenerated cause problems. You can tell when the diesel is regenerating, you can get a slight smell of diesel in the cabin (for a matter of seconds, as more diesel is being pumped into the engine to make it run hotter, the water temperature rises and the radiator fans tend to run when the engine is switched off. I always use a DPF cleaner every 2nd or 3rd tankful of diesel.

I suspect that is just doing short journeys John. I doubt stop/start would make much difference to DPF clogging.