Mercedes SLC Undervoltage Issue

Hmmm, undervoltage on a 2020 model is unusual, though not impossible. You say you checked the battery terminals, but did you measure the voltage drop across them when the engine is cranking? A bad connection can look fine but still cause significant resistance under load. Also, has the alternator been tested? A failing alternator will definitely cause low voltage and drain the battery. Low voltage readings during operation could really point to alternator issue. Otherwise, parasitic draw could be in play, slowly draining the battery while the car sits, but that's harder to diagnose.

I haven't measured the voltage drop specifically, just visually inspected the battery terminal connections. How would I accurately measure the voltage drop, and what's a typical acceptable range? Also, if it's not a parasitic draw, how can I check the alternator output effectively to rule that out as a cause of the electrical problem?

Measuring voltage drop requires a multimeter. Connect it across the terminals while someone attempts to start the car. Ideally, you want to see as close to zero volts as possible – anything more than 0.2V indicates excessive resistance. Alternator testing usually involves checking the output voltage with the engine running. It should be between 13.5 and 14.5 volts. If you're not comfortable with electrical testing, I strongly advise visiting a workshop. Sustained low voltage can damage sensitive electronics in the car.

Thanks for the advice. I took it to a workshop; turns out the battery was indeed defective and heavily discharged. Replaced it and all is well. Final repair cost was 290€.