When the alternator cannot handle the demand placed upon it' date=' and the voltage falls to the battery's domain: The battery "reserve" will help voltage @ this point. Not above it.
During music, since the amount of draw is varied: If the alternator "falls behind" to the point of batteries being used, the battery will discharge until the music selection reaches a point that the draw demand is less. Then the alternator will be the source again and be taxed by draw and charge the batteries.
Tried to keep it as casually worded as possible.[/quote']
That’s all true but the op was asking what he is doing wrong because he is seeing his voltage drop with a stock alt trying to keep up with everything plus two extra batteries. Those extra batteries are costing him about 20amps of alternator power 10 amps each. I don’t think the op said whether he was playing music or doing a burp when he was seeing his voltage drop. It kind of sounded like he was talking about a burp. He was also talking about what his coils are wired to but not taking box rise into consideration. Maybe the op can clear up whether he is playing music or burping. Extra batteries on a stock alt is all good for burps but not doing much good for daily driving use.