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 
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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.
. New one ordered off eBay.
