Stereo & Electronics - bad battery, alternator, or what?

Today i while i was driving i noticed my radar detector kept making weird beeping noises at me. Later on i noticed on my battery gauge next to the fuel gauge that when i started the car it would be in the red. After driving for a while it would hover between the red and 13. My radar detector worked fine after while it hovered between the two. I just got a new battery in May and since its acting weird while its running I was thinking maybe the alternator. Also, I have the ASP crank pulley but not the alternator pulley. What do you guys think?

I have been told that the alternator has 3 diodes. After the first one fails you have what you see, 12 or so volts, and in the next few days the voltage will go down to 8 or 9 and a couple minutes later the 3 diode will fail.

Yes Get the alternator pulley it will overdrive the alternator. The problem is when its idling is that its trying to pull more amps then the alternator can make.