If you charge the battery, you don't have to worry about the voltage you are delivering at any given time.
The battery is 12.6v and is normal, the other thing is that it does not turn on when it is working, it works normally but what worries me that it drops to 12.9v.
I measure the direct voltage in the battery and accelerate it to 2000rpm and go up to 13.4v then go down to 12.7 or less and it is very unstable. That alternator has 8 years with me and before I delivered 14.2v these days I measured it and it was stable at 13.4v and well now it is unstable.
You have to take into account the state of the battery, if you are more than 3 years old it may be low and you want a change.
Right now I will disassemble it to see what fails I see it. and thank you all for wanting to help me friends.
Discard problems of poor contact in the battery when measuring.
A possible fault is a wear of the brushes.
You have to check diode holder plate carbons rotor rings that are smooth regulator check all welds the load must remain stable if it is stable your lights will go up and down
If the voltage at the battery terminals reaches 11.8V, it may be due to bad contact. Otherwise it should not drop below 12.5V unless the battery is bad.
Friend, I corrected all the welding of diodes and the regulator, now it is maintained at 14.0va 2000rpm and at 1000v it lies at 13.8v. It is an acceptable margin. Thank you for your suggestion, very grateful friend.