Amp question. ok on my sub amp it has a digital voltage readout that i mounted under the dash.

Question

ok on my sub amp it has a digital voltage readout that i mounted under the dash. with my car running the voltage for the amp only says 14.0 or 14.1 this is with everything turned down. now when the car is off and i crank it up the voltage will drop below 12.0 even 11.8 its loud but i know im losing alot of spl. what should i do to correct this add another yellow top battery in back? could somebody explain how too hook up a battery in the trunk? thanks