Mercedes B Klasse Diesel Battery Drain Issue

Sounds familiar. I had a similar discharged battery problem with my old Opel Astra, though it was petrol. The symptoms were almost identical: low battery warnings, lights dimming, and eventually, a battery that wouldn't hold a charge. In my case, it turned out to be a faulty voltage regulator. Have you checked the alternator output? A bad diode in the alternator can cause a parasitic drain, leading to a low battery even when the engine is off. Also, how old is your battery? Sometimes it's just an old battery reaching the end of its life.

I haven't checked the alternator myself. I'm not very technically inclined. The battery is about four years old, I think. How can I check the alternator output myself, or is it best to leave that to a professional?

Four years for a battery is getting up there, but it shouldn't be completely dead unless it's been abused. Checking the alternator requires a multimeter and some basic electrical knowledge, so if you're not comfortable, I'd advise against tinkering. Based on your symptoms and the error code, the 'defective battery sensor' diagnosis might be spot on. Those sensors can cause all sorts of weird issues with charging and voltage regulation. If your workshop suspects that, it's probably worth having them replace it. It's a relatively common problem. I would recommend you get it checked ASAP. I wouldn't want you to end up completely stranded. Driving with a discharged battery isn't ideal either.

I took it back to the workshop, and they confirmed it was indeed the battery sensor. They replaced it, and the total repair cost was 315€. Everything seems to be working fine now; no more error messages or weak lights. I really appreciate your help!