2005 S80 T6 New battery and alternator, battery light
 

Can anyone help!!! Okay so I bought a 2005 S80 T6 with 57k on it, however when I purchased the car the battery light was on and the power system urgent text was on as well. It drove well, very quiet and smooth, no studder or anything and I drove it an hour home. After a couple days ,it started to show reduced engine performance and would studder and stall. I was able to pull over for a few minutes and get it started to drive home. After a few days of that, the alternator completely died and the battery wouldn't hold a charge. So I recently got a brand new alternator and battery put in and the car was almost as good as new, except that the battery light and power system urgent text was still on but the car starts right up every time with no hesitation. Well I went for a spin yesterday and when I got back into the car to go home, upon acceleration the car had barely any throttle response and the reduced engine performance text came back on, so I coasted to the side of the road, turned the car off for a few minutes and started her back up and it drove all the way home as if nothing happened, but the battery light stays on and power system urgent text stays on as well. Anyone else have this problem and if so, how do I make it stop?

You are shooting in the dark unless you have an appropriate scan tool to diagnose the problem. I am pretty sure that for 2005 the ECM controls the alternator output. So, some aftermarket alternators might work but only charge at a fixed 13.5 volts.

Start with codes. If you have replaced the alternator, you need to be sure that the one that you installed is compatible with an ECM controlled voltage regulator.