Damn! I was always thought that all variants of CLA came with an AUX battery for ancillary functions like Eco Start/Stop (though I always keep eco off). However, I was so wrong when one morning my CLA 220 (2017 model, manufactured in mid-2016) would not crank at all. The battery voltage went so low that even car doors won't open using key-fob. Anyhow, I was able to jump-start and went straight to the authorized service centre.
On my way I was thinking, why did the AUX battery not enter the circuit if the main battery is down. Never a warning on the dash, nothing. Diagnostic engineer at the service centre said that my main battery needs to be replaced as it's 2016 made. Took $365 all-inclusive to replace the main battery. When I asked them about AUX battery, they said my car did not have one!
Later that day, I checked the footwell to confirm their claims, and indeed there was no battery, rather a small black box with part number engraved A2465409611 which houses 'VOLTAGE DIP FUSE'.
I did a little more research and came across this article which talks about how Daimler engineers were able to achieve the target of reducing cost and non-reliance on a backup battery without compromising on the efficiency of start/stop function. The article is called "Ignition current limiter replaces the backup battery concept"
I personally think it's very cheap move by Daimler. My Beamer never had this issue as core electrical always worked when either of the battery is in good condition. I wish my Benz had an AUX battery too!