W212 Identify battery or batteries?

[Beemer2]

I have an E250 W212 Premium AMG Sport Petrol car that has stop/start and keyless go. I had a problem where the stop start was not working and I suspected a low battery voltage as a result of the car sitting idle during Covid 19.
I put my Accumate III charger on and it stayed at orange for three days then went green. Still the Stop Start would not work but I learned that for a AGM battery the charger only took the capacity to 80% when the green charger light came on. So I left it another day and the off load voltage rose to 12.7 Volts. Now the stop start works. What I'm curious about is that online I see several configurations of battery systems and locations for the W212 but none like on my car. This has led me to now ask what is the configuration on my car. Here are photographs that I took today:
I guess the main starting battery is the one in the boot. Is the one in beside the engine a stop start battery or a capacitor?
I have never seen a battery 160mm Long, 42mm wide and 220mm deep.

The battery in my boot looks just like the auxiliary battery I see in some YouTube videos.
So if it is where is the main battery. It certainly is not in the engine compartment unless the strange sized one is it.
I looked under the rear seat as that is another place that Mercedes put a battery but there was nothing there.

Ian

 

[alecmascot]

That's a pre-fuse/charging/jump start connector.
Look under the large cover just behind it.

 

[Beemer2]

Okay now I feel stupid not noticing the yellow battery label! I released two catches at the rear left and right side of that cover but it will not budge. Can't figure out what is holding it but perhaps you know?

UPDATE: I now see there is a center catch.

 

[SW18]

That's correct, the main battery is under the boxy black cover at the rear of the engine bay in your photo. My set-up is the same and my main battery is one of these (other makes are available, not sure what the OEM make is):
yuasa ybx9115 - Google Search