Alternator wasn't charging the battery after 100k miles on my wife's car, so I replaced it. A month later the exact same issue is happening. I did a little more digging:

​​​​​​Idle voltage is 12.4-12.6V
Voltage while driving @1500 rpm is 12.4-12.6V
Voltage while coasting to a stop before the transmission shifts down and my foot off the gas peddle is 14.5V.
Voltage after the transmission shifts down and I come to a stop is 12.4-12.6V.
Anytime I'm driving and I take my foot off the gas peddle even for just a few seconds the voltage shoots back up to 14.5V. Then, when I put my foot back on the gas peddle it drops back to 12.4-12.6V.

I'm 42, boring, and I don't hotdog the car. It's bone stock. Any clues?

 

Current sensor. The little black box thing that attached to the negative battery terminal and has a plug-in going to it. Cost about $40 on rock auto. As soon as I changed it out, which took about 2 minutes, I had full voltage again. 14.5V.