I have an aftermarket system in my truck, and since putting it in, the volt meter after about 5 minutes, will dip down to about 11.5-12.5. It's used to be around 14-14.5. I also have a 1 farad cap wired into the system. Do I need a higher amperage alternator? What one should I use? This is in an 05 Sierra extended cab z71. I believe that it is a 145 amp alternator in there now. Should I also upgrade the battery?
Thank you!!

 

I assume your reffing to the stock voltmeter? The meter will fluctuate some as electrial componets are turned on and off, and the stock meter isnt 100% accurate. what is the current draw from your amp? 11-13 is normal operating range of voltage

 

The readout on the cap will start around 14.43 then after about 5 minutes, it dips down to about 11.8-12.1. I don't notice lights dimming or the problem I used to have with the amp going into thermal protection, just noticed the stock voltmeter now reads lower than before and it's pretty much verified by what is showing on the capacitor.
The meter will fluctuate when no other components other than the stereo on.
I listen to heavy metal music with alot of continuous double bass drums. Maybe it's putting just a bit too much strain on the alt trying to keep up with the current draw demand?

 

ive got a similar set up. got a 1 farad cap and it hardly ever dips below a 13. it comes on at 14.1 and that is listening to heavy bass music.