I have tired myself for months trying to figure out the problem. First, let me start off by saying that I just recently tore my whole dash apart and did all that happy stuff evolving a new a/c heater core, so if that info helps...here we go.

My car died just recently and I thought I just exhausted the battery on the car trying to fix and tune some stereo and miscellaneous parts on the car. I figured it would be a good investment to by a good battery, so I did. After the new battery installation, I ran the car for a while to see if everything was ok. It wasn't, my battery gage was around 12 volts and testing the battery with a volt-meter, it was running at about 11.5, and the gage wouldn’t really move at all when running the car. I thought it was a bad connection with the wires behind the instrument panel. I tore the IP apart again and checked the connection, which was good. So then I thought it was my alternator. I took my old one off and inspected it, and it caught when it was spun freely in a small spot. I thought what the hell, I should get a good alternator for the car too, so I bought a new one (which I kind-of regret now cause the old one tested fine). Hooked the new one up and still the same results as before. So then I went and tested all the connections on the alternator, battery, coil, and found nothing out of the ordinary. I also tested all the battery cables, wires to and from battery and alternator. I replaced the pigtails coming off the alternator thinking corrosion was the cause of the bad voltage, and still, the same results.  I have tested all the fuseable links i could find and they all tested fine.  I have tested all the wires at the starter relay...fine.
i replaced both battery cables and checked the one from the starter to the relay...all good and yes...it helped....to a point...
the car sits around 13.5ish if not a little higher/lower.....starts, and dips like it should when it cranks...
when its running though, it creeps up reeeeally slow, and once it gets back to about 12.9-13.4ish voltage, that is where it sits.
granted i know that voltage is OK...but i dont like it bein OK. I would like to fix the charging problem so i know it is reliable. and if the car sits for a couple days it completely dies...

tested the new altenator and it is fine as well (brand new battery too)

I am running out of ideas...and help would be greatly appreciated!!

ps...nothing really electrical done to it except a MSD coil, autometer tach, and a head unit (no real system, just basic speakers to replace the stock ones and a new deck)

I am also trying to confirm the general location of these grounds:

- engine block to firewall
- motor mount to frame
- battery to engine block (i know this one), radiator support and dash harness ground point.
- dash harness ground point to frame  

Regardless of the voltage output issue the battery shouldnt "die after a couple of days sitting". You may have a short and need to check the "OFF" amperage draw with all circuits in the OFF position. Anything more than .5 amps is too much and you're draining your battery.

You also need to have an in-car test performed to see if the alternator amperage is per spec since that's what really charges the battery. The voltage is controlled by the regulator but if there's not enough amperage, your battery is going to die.
Keep us informed.