Ok Jaguar family I got a 2008 X-Type 3.0 while I'm the "test mode" with engine running the "BAT" is reading 12.3 that's with the headlights on, radio, and AC running, and heated seats on, when I turn off those things it went up to 12.8, when I first turned it on not running it was 13.2. I did this while sitting still reason is my battery light comes on but when I shut the engine off and start it up again it goes off. The last time it happened I had to change alternator that was about a year ago replaced the battery a year before that oh the alternator issue was that it became frozen for some reason and was spinning at that time the battery was good. Sorry for long post just trying to give good idea of situation. Which one should I replace first lol

 

Neither at present .........until you do some further tests.
Read post #4 of link below for a bit of insight on the charging behavior.

https://www.jaguarforums.com/forum/x...normal-208377/

If you have replaced the alternator with a genuine part, then it is highly unlikely that it is faulty after a year. If you fitted a generic alternator, all bets are off as often they do not interpret the Jaguar ECU commands the same way original alternators do.
The battery likely is not faulty after a year also, but that might depend on battery type, capacity and storage factors.

Worth considering battery cables though..... as hidden corrosion of wire strands (beneath wire sheathing) can severely degrade starting and charging performance.

Good luck.

 

+1 for Mark's comment about the battery cable. It is a quick and easy test to see if cables are getting hot meaning they should be changed. Bad cables are popping up on this forum a lot.