Mercedes SL battery drain issue

Sounds familiar! I had a similar problem with my older S-Class. A discharged battery and flickering lights definitely point towards a battery problem. Before replacing the sensor, have you checked the battery itself and the terminals? A corroded connection can cause weird issues, including preventing proper charging. What's the battery voltage when the car is off? That can help isolate the problem.

I inspected the terminals and they seem clean. Where is the best point to test the voltage and what reading might confirm the defective diagnosis?

You can check the voltage at the battery terminals with a multimeter. A healthy battery should read around 12.6 volts with the engine off. If it's significantly lower, like 12 volts or less, even after charging, the battery might be the culprit. Given the error code, it's likely the sensor. Replacing the sensor is probably the next best step. A workshop visit is in order to confirm and replace it.

Based on your recommendation, I had the battery sensor replaced at a workshop. The issue is resolved, costing 315€.