I have a 2007 S60R M66 with 167k miles. Bought the car in March of 2021, enjoyed a fun summer, and had her parked for most of this winter because I don't have working AWD. Exactly a month after I last drove her, she started up nicely. A few seconds afterwards the warning "Power System Service Urgent" appears in the message box, along with a red warning triangle and battery light. It also seemed as though the idle was a tad lower than normal. I took the car for a short drive to see if things would clear up as they sometimes have, the ABS light came on with a Brake System Failure notice. In reverse, the dash lights dimmed but resumed normal brightness when taken out of reverse. On the way home she gave a good solid backfire for every shift, which was unusual. The battery was at 11.6V when the vehicle was off. I could take a good guess that battery was older but putting a new one in didn't fix the problem. While the vehicle was running, the new battery showed 11.7V at idle; turning on accessories would drop the voltage. The warnings on the dash would turn off and come back on a couple seconds later every few minutes or so. My code reader picked up relevant codes. ABS Module Voltage Too Low (0057 - Not Active), Alternator Control Module Electrical Fault & Mechanical Error (1C06 & 1C08 - both Not Active), Central Electronic Module Power Supply Signal Too Low (1C05 - Not Active), and Communication with Alternator Control Module Signal Missing (DD30 - Active). Curious to see if anyone has had problems like this, I want to be able to diagnose this properly.

 

Reading your description, everything is pointing to the alternator, its voltage regulator or even the clutch on the pulley. If it's original, it may be time for replacement of components or the whole thing