Problem is your working voltage is 14V and your reserve is 12V.
So your batteries are not @ 14V with the car running....thats not correct....You know this.
Every time you dip into that reserve, your system voltage drops. Not a problem, per se, but dimming lights is really annoying and the additional load of the headlights makes this more likely to occur. This is why capacitance is nice IMO because it is reserve as well (given not as much as a battery but you will never use 95%+ of the battery's reserve anyway) and it will start at 14V and slowly decrease to a lower voltage. The lights will still dim but it will be a more gradual fade rather than a sharp flicker, ie no disco effect. Once the voltage regulator on the alt notes the drop in voltage, it ups the current output and all is happy again. For stopping headlight flicker with music, a battery will simply not help..
Ok so you think a battery has no capacitance? What do you think fills a capacitor ? When you first charge one - is the car usually on ...... No. A battery and cap are similar - one holds a reserve and by adding a second you doubled the reserve (assuming both batteries are equal) Adding a cap just adds capacity to hold power that is already in the system. Its like an air tank for airbags - it did not make the air but will hold it and release upon demand.
A battery will certainly stop any disco light or slow fade. You have to look at the whole picture, not just one part. Everything works as a system. With the car on the entire system operates at 14V. You cant say that this burst came from the ALT or this low note was powered by the battery (unles car is off, we know that) All parts are daisy chained together and therefore react together.
Of course low ESR works on both the discharge (good) and charge (bad) when the alt does catch up, a battery of small caps with relatively lower ESR will present a larger current load on the alt. .
Again, second battery - the ALT does not need to catch up. You see the ALT and batteries are directly connected, so while the ALT creates the 14V operating voltage, large spikes come from a battery and the ALT adjusts to reload for the next burst. More reserve means less to reload.
On a large engine this load is nominal, and the load is still in proportion to the demand placed on the alt. The only difference is that the potential of the alt is available at a lower engine rpm. .
I meant it is harder on the ALT but it will add a load to the engine, if you do over drive your ALT it could cause premature failure. If in a car that is taken to redline often, you could possibly exceed the bearings rpm tolerances (this is not likely, but in extreme cases like we seem to be talking about - could really happen) Bearings go on units all the time just from time, excessive heat could break the grease down premature.
You have a system in your car - turn it on start your car and play some music. See what the voltage does at the front battery (not of a block or cap - a real DVM on the battery). with MUSIC it will bouce around, but never drop conitously. Have a friend pull up next to you, connect his battery to your car with jumper cables. Let your car run for 30 seconds with his connected to make sure both batteries are equal - see what happens to the voltage now. Its easy and free learning //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif