It’s your alternator which has an internal regulator....if it is the original equipment....it’s almost 20 years old
Time for a new one....
Thankfully it’s a Nissan, if it was a Chevy, you already be on your second or third replacement...
My 1995 Nissan is on it's third alternator; thankfully, it's the only "mechanical" expense I've had in 19 years. As a result, I buy my Nissan new "presents" every once in a while. :)