Don’t forget to check connections at starter as well; a little bit tougher to get to & ensure you have a good ground. Extra ground leads don’t hurt & for the record...12.60 is a fully charged battery...14.5volts while running would be about the max voltage you’d want it set to. AGM batteries prefer closer to 14.0
Ck idle rpm; as TB’s or Maf’s may need cleaning & don’t forget about a quality serpentine belt...good insurance to replace every 50-60k

I had Nissan "check the wires and connection". The serpentine belt looks good was thinking of replacing it soon. I haven't driven the car for 2 weeks and now the car is runnimg closer to 15v, with a drop to 13 at stops.

Try this... Take your voltage meter, set it to A/C and measure it across the battery while its idling. It should not give you a reading (take it through all of the scale settings). If you get a reading, then its a faulty alternator

i have the same problem at the moment as well. If I start the car, it idles at 14V. I can leave the car idling for 15 20 minutes and the voltage wont change. However if or when i start driving the voltage will fall to 12.6V as well and i could drive over a bump or something and the voltage will move around.

Possibility:

Im investigating the thought that it could be a slipping Serpentine belt due to a loose tensioner. il come back to you on this.

Looks like it was a battery issue for me. One of the cells was playing up and thus the intermittent nature of it. my cranking output was also half now due to this issue. Luckily it was under the battery manufacturer warranty.