This is possible that this is normal but we need to do some testing to find out.
We need to perform two tests to start. First is to test the alternator and then the battery. The fact that this is a used battery is concerning but that may not be an issue.
Also, are there any issues with performance or is this just a concern you are having because you are seeing it drop to 12 volts.
This charging system is on what is called a duty cycle. Basically when the load or demand is high the alternator puts out a higher voltage.
I attached all this info below on how to check this and the chart that shows the approximate output voltage based on the duty cycle.
Please run through all this material and let us know if you have other questions.
Images (Click to enlarge)
Wednesday, July 21st, 2021 AT 2:22 PM