If I have my headlights on, foot on the brake pedal, and have it system al full volume, my volt gauge will sometimes touch the 9V area.
I did some testing on my car to see if this had a big effect on the system.
I used a few batteries in parallel to increase the current supply, and I found that the extra batteries only helped out when the engine was under around 1200rpm, with headlights
on etc. Otherwise, there was absolutely no audible difference.
I have had my setup in for 3 years now, and only spent $125 on a battery in that time, and $40 on my alternator.
CAUTION: Slippery When BOOSTING!