Sorry for the delay guys and gals, My second battery is run just like firedevil said
i just ran 4 gauge from the positive of the first batt to the positive of the 2nd batt. then grounded the 2nd batt in the trunk.
Mine is 2 gauge, but the same, DO makee new bigger grounds off both batteries to the frame, back seat battery is grounded to the big 25mm seatbelt bolt that goes thru alot of metal. trunk is to any clean spot that'* convient.
Mine is Optima Yellow top. Don't buy a red top for a 2nd battery, there designed for a cars main battery, mine is still stock for now, will look into a red top, but not sure if it will fit in the slot under back seat. Anyway not up to date as what the blue tops where designed for, think Marine use.
As for an Alternator up-grade, I'm sure it'* a good idea but I don't have one yet, car only has 26K on it now, when it goes it goes. Then I'll get a 200 or something higher
Caps, I do recomend them for +1000 watts of bass, won't hurt you especially with 2nd battery, but I bet not neccesary, I do think the new style of caps like mine are better than the old stiffing caps aka battery style cap. Mines 5-farad from alumnapro.
With my setup my DIC voltage is almost allways 14.7 even with all assesories on and bass pumping. I check the voltage off the amps and they also match the DIC.
When the batteries get drained from listing while off, over maybe 1hour or so comes down into the mid 13'*. Takes good hour of driving to get them back to 14.7 but when it gets there it stays.
Interestingly talking about Alt'* and voltage and stuff with my installer buddy. He has been reading about the advantage of voltage-regulators and jacob electronics sells these, orignaly designed for race cars. What ever the amps it gets it puts out the set amount of 14.7 volts or what ever you want. 13.5 or say 15.2. These will increase amps power big time. Not sure though If I can put inside my car, they got hot and should be under the hood, need to write Jacob'* and see what they say. Topic came up as dosen't matter how many amps you make, how much constant voltage are you getting. This sounds like a good idea and am looking into it, I know that hardcore SPL guys have been using these for years, that amps gotta get x amount of voltage all the time to produce X amonunt of DB'*
Anyways, hopefull I answered the orignal question and them some. Any more?