Problems with my Audio Capacitor?
 

Hi I was installing my new sound system, 2 Alpines Type-E 12", Amp BOSS 1600w and a 3.5 farad Planet Audio capacitor. Now, I connected everything.

Positive:
Battery -> Capacitor -> Amp

Negative:
Capacitor -> Ground
Amp -> Ground

The amp turns on and give power to my speakers, but the voltmeter from my capacitor does not turn on. I TRIED to charge it but well I think it didn't work, did I have to charge it connected to the amp? or without the amp? I don't know if this matters, but both grounds are in the same screw.

What could it be? Any suggestions?