Power System Service Urgent + battery light

[DianaPrince]

2006 S60 2.5T

Replaced alternator and battery recently. Was randomly still getting the “Power System Service Urgent” message and the battery light would illuminate at the same time; both would only stay on for a few moments then go away. Now both the message and the battery light are on constantly. Worried that the alternator was broke again I read the battery with a multimeter: with engine running it was 14.75; with engine off it was 12.89. This seems normal, yes? So why would the power system message and battery light still be on??

[P80GLT]

Did you replace with the correct alternator. 2006 should be a computer controlled system and earlier alternators will throw all sorts of faults