Mercedes S Klasse Hybrid Auxiliary Battery Issue

I had a similar issue with my BMW 5 series, though it wasn't a hybrid. The symptoms sound familiar. The auxiliary battery system in modern cars is complex. A voltage drop in the auxiliary battery can trigger seemingly unrelated errors because the car's power management system interprets it as a system-wide issue. Have you checked the charging voltage of the auxiliary battery itself? It might be a faulty charging circuit rather than the battery being completely dead. Also, check the connections for corrosion; a bad connection can cause a significant voltage drop. What's the exact error code you're getting?

Thanks for the reply. I haven't measured the charging voltage directly at the auxiliary battery, but that's a good idea. I'll check the connections for corrosion too. I'll get the error code from the diagnostic report. Could you explain how the power management system interacts with the auxiliary battery and triggers widespread errors? I'm not familiar with that.

The power management system monitors the voltage of both batteries. Low voltage from the auxiliary battery can trigger warnings and limit functions to conserve power, because many systems rely on a stable voltage. It's a cascade effect. If you're getting generic low voltage errors with specific module errors, the auxiliary battery is the main suspect. Honestly, being a 2011 model, the auxiliary battery is probably nearing the end of its life. I'd recommend a workshop diagnosis and battery replacement. In my case, the battery failure was the root cause.

I appreciate the explanation and advice. I'll get the auxiliary battery and charging system professionally diagnosed. Given the car's age, replacing it preemptively seems wise. The workshop found the auxiliary battery was faulty and replaced it for 290€. Thanks again for your help.