That may be the best way to run them, I've never had them before but if I can wire them off my cars batt. under the hood directly they may not dim. I wonder how my headlights are wired now, come to think of it? As the alt. and audio batt. should be taking all if not 90% of the audio power needs, thus leaving the under hood batt. to focus on powering hids. Sounds good to me when I read it back to myself.
If you have the sound system wired to the second battery and then have a special charging device that limits how much amperage can be drawn from the mother system (alternator and battery combined) then it will have little effect on the mother system's components.
If, however, the connection from the second battery to the mother battery is simply with a cable large enough to act as a jumper cable, then any load on the second battery is instantly shared by the mother system. If that load is so large that your lights dim then just imagine what they would have done without the second battery!
Both batteries are wired in parallel. If one of the connections (pos or neg, doesn't matter) is attenuated by a device that limits the current passing through it, it can be limited both ways or only one.
A diode is like a one-way valve for electricity. If you think of it like a hose then this "valve" can let all the "water" pass in one direction with no restriction but reduce or prevent it from flowing in the other direction. A diode does the same with electricity.
So if there's a device that doesn't limit the flow in one direction (ie while charging the second battery) but limits it in the other direction (ie while starting the engine) then the second battery will be a parasitic draw on the mother system but won't supply any real advantage to any of the mother's components.
I hope this is understandable!
