S80 Auxiliary Battery Error

Sounds like your auxiliary battery isn't charging properly. Have you checked the voltage while the engine is running? It should be around 13.5-14.5V. If it's lower, the alternator might not be doing its job, or there could be a parasitic drain somewhere.

I haven't checked the voltage myself, I don't have the proper equipment. I guess I could buy a multimeter, but where exactly do I check? Is it like checking the main battery voltage, or is there a special spot for the auxiliary one? Also, what does parasitic drain mean?

Yes, you can check it the same way as the main battery. If the voltage is normal (around 14V), the battery is likely dead. Think of it like a leaky faucet, a parasitic drain is when something is constantly drawing power even when the car is off, draining the battery. Given the error message and weak start, it's highly likely the auxiliary battery itself is the problem. I would recommend bringing it to a workshop.

Thanks for the help! I took your advice and went to a workshop. They confirmed it was the auxiliary battery, not charging, and replaced it. Cost me 290€. Everything seems to be working fine now!