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[LC0112G]

They do!

The Merc system monitors the battery voltage drop during cranking. If the voltage drops below (IIRC) 11.5V then it disables Stop/Start. It'll still start all the way down to less than 9V.

It's not really the state of charge that matters, but the batteries internal resistance. The resistance increases as the battery ages, so it can be fully charged (>13.7V) but still drop enough during cranking to disable the Stop Start.

Mine's been like it for over a year. Behaviour confirmed by iCarsoft scanner. New AGM Stop/Start battery would no doubt cure it, but why on earth would I want to do that! £300 to re-enable something I don't want.

If they do then they are fools! (I am confident they don't as we use the same setup!), they wouldn't meet ISO 16750-2 under those conditions

Monitoring the terminal volts under load tells you little about the battery condition as there are to many variables, temperature being the very significant one, as the chemical reaction in the battery is much poorer at low temperatures and so the VBatt drops much lower. There are also variables with connector/cable ageing, engine types and a whole host of other things.

The SoC is the critical measure for start stop, as it is the only means to tell if you have the reserve to complete repeat re-starts, its also the only viable means to determine battery health by checking the batteries ability to absorb energy 

If you google "Mercedes eco-start-stop-technology-guide-pdf", and click on the link to the glaowners.com, you'll get a pdf describing the system. There is a whole load of guff in there, and I checked it all on my car. The only thing that is out of limits is the battery voltage drop.

What I'm unsure about is if it's the drop on the small auxillary battery, or the main 'big' battery. But I can see on the iCarsoft that the 'battery' voltage dips to between 10 and 11V on my car (from > 12.5V) , and that's enough to disable stop start.

[Doctor Gollum]

The R had the diddy one next to the main battery under the front right seat...

[Andy B]

The R had the diddy one next to the main battery under the front right seat...

The QR code on the inside of my fuel filler cap suggested that my R Class had one there too ..... it didn't though 

[Doctor Gollum]

Possibly spec related

[Marks DTM Calib]

If you google "Mercedes eco-start-stop-technology-guide-pdf", and click on the link to the glaowners.com, you'll get a pdf describing the system. There is a whole load of guff in there, and I checked it all on my car. The only thing that is out of limits is the battery voltage drop.

What I'm unsure about is if it's the drop on the small auxillary battery, or the main 'big' battery. But I can see on the iCarsoft that the 'battery' voltage dips to between 10 and 11V on my car (from > 12.5V) , and that's enough to disable stop start.

That's not what the document says at all.

If the voltage dip (U < 11V) at engine start is to great, the engine stop is deactivated for t = 1 minute in order to recharge the Eco start/stop function battery

Its not actually defining what it considers to great and it also is only delaying activation of the stop start for 1 minute. The rest of the doc is referring to 'available electrical power' which is the SoC.

Also, be absolutely assured that your scan tool will not be able to report the mS long volt drop at cranking as its just reading messages at random intevals, you need an oscilloscope on the starter terminal to do that. The volt drop will be down to 9V or less on a cold engine crank (remember, this is my day job on the design side  )

[LC0112G]

If you google "Mercedes eco-start-stop-technology-guide-pdf", and click on the link to the glaowners.com, you'll get a pdf describing the system. There is a whole load of guff in there, and I checked it all on my car. The only thing that is out of limits is the battery voltage drop.

What I'm unsure about is if it's the drop on the small auxillary battery, or the main 'big' battery. But I can see on the iCarsoft that the 'battery' voltage dips to between 10 and 11V on my car (from > 12.5V) , and that's enough to disable stop start.

That's not what the document says at all.

If the voltage dip (U < 11V) at engine start is to great, the engine stop is deactivated for t = 1 minute in order to recharge the Eco start/stop function battery

Its not actually defining what it considers to great and it also is only delaying activation of the stop start for 1 minute. The rest of the doc is referring to 'available electrical power' which is the SoC.

Also, be absolutely assured that your scan tool will not be able to report the mS long volt drop at cranking as its just reading messages at random intevals, you need an oscilloscope on the starter terminal to do that. The volt drop will be down to 9V or less on a cold engine crank (remember, this is my day job on the design side  )

The scan tool does show the drop - it plots a nice little graph. AIUI the SAM measures the voltages and stores them in a ring buffer at a high-ish speed. True the scan tool only 'samples' the data at a slow rate (<5 per second), but the SAM reports the minimums/maximums observed in that interval. So you can see the dips and peaks in the voltages.

You can't directly measure SoC either. All you can measure is current draw and terminal voltage - and the Merc system does both. From those two values you can estimate SoC (by integrating the charge/discharge rates) - but these are derived, not measured values.

What I *Think* happens is that during cranking, the car uses the main battery for the starter motor, and a few critical other systems. However, non critical systems are switched over to the Aux Battery during cranking, and it's this battery that is showing the voltage drop. If it drops to < 11 V then Start/Stop is disabled (t = 1 minute in order to recharge the Eco start/stop function battery).  However, if the battery does not recharge to > 13V in that 1 minute period, then...

If the open circuit voltage (U > 12.5V) of the ECO start/stop function additional battery is too low, the alternator's output limitation (alternator management) (except model 212.095) is deactivated so that the ECO start/stop function additional battery can be charged.

If the voltage dip (U < 11V) at engine start is too great, the engine stop function is deactivated for t = 1 minute in order to recharge the ECO start/stop function additional battery.

The ECO start/stop function additional battery is only connected if the output limitation of the alternator is not active and the on-board electrical system voltage is high enough (U > 13V). It is disconnected again if the limitation of the alternator is active or if the on-board electrical system is severely overloaded.

So if the Aux battery dips < 11V and then doesn't charge to > 13V, stop/start is disabled. It doesn't say it explicitly, but experiments seem to show that the Aux battery needs to charge to > 13V within 1 minuite, or Stop/Start is disabled for the entirity of that engine start.

[Olympia5776]

I'm finding the stop / start function is a Real pita , I really don't want to have to go through a pre flight check to manually disable/enable it ,the lane assist and gradient handbrake each and every time I use the car .
It took me nearly two hours to figure out how to switch the duplicated map on the dash in front of me ( which was already on display on the full central screen)  showing me where I was .
I'm nearly where I want to be regards all the features but that's a week of 'kin headache .
Owners forum gives great feedback on this so it is winging it's way to me as I write this .

https://www.aliexpress.com/item/1005003954198774.html?spm=a2g0o.productlist.main.1.a6f922bcLOy1S0&algo_pvid=6beddbd1-3a3f-4361-af2d-569dd34ffb4c&algo_exp_id=6beddbd1-3a3f-4361-af2d-569dd34ffb4c-0&pdp_npi=4%40dis%21EUR%2121.10%2110.31%21%21%21161.04%2178.68%21%402101ec1a17069142707876675ecb9a%2112000027625874088%21sea%21IE%214668763959%21AB&curPageLogUid=ufzyDQ5JHw1a&utparam-url=scene%3Asearch%7Cquery_from%3A

[YZ250]

I'm finding the stop / start function is a Real pita , I really don't want to have to go through a pre flight check to manually disable/enable it ,the lane assist and gradient handbrake each and every time I use the car ……………….

I daresay manufacturers will try to block over-riding stop/start but I’ve coded mine out on both of our Audi’s. On VAG vehicles it’s really easy to code out using VagCom, or Carly, Carista, OBDEleven dongles etc.
If not possible to code out, people would set the stop/start battery parameters ridiculously high using the above dongles so that stop/start voltage could never be achieved.