I bought 2 Optima Yellow Tops today for less than $30 a piece. I was going to mount one in the trunk and sell the other.
But i dont want to put the draw on my alternator. The alt will try to charge both batteries and will put a hurtin on it.
Was thinking about an isolator. So my system only pulls of my aux battery but wont help much cause my system will only be running off my aux battery, there for, my Volts wont increase so the Aux batt will be pointless unless i run my system with the car off. then only my aux batt will drain and not my main batt.
Capacitors are useless..a aux battery provides over 10x more stored power than a Cap..
So im kind of stuck... I cant afford to upgrade my Alt at the moment...
Anything im missing? any ideas?

it won't try to charge it unless it'* been drained.... a fully charged battery doesn't need a whole lot of amperage to bring it up to the alt voltage, only something like 1 amp or less...

and if you're gonna draw power, you're gonna draw power, regardless of the source, might as well have double(or more) the reserve to try and buffer the alt to keep system voltage up.

The purpose of a cap is for those short bursts where a battery cannot put out the power quick enough. I do not have a cap in my system though, just a second battery. The guy that helped me design the system also said that caps are worthless.

I put an isolator on mine and set it up so that when ignition is off, it runs off the second battery. This keeps the two batteries from fighting when the car is off and also prevents you from running both dead and not being able to start your car.

I also had to put a button in to manually trigger the isolator with power from the main battery because I found that when I ran the secondary dead, it didn't have enough power to flip the isolator and turn on the main battery... doh.

I'm running with a stock alternator, but I do have upgraded wiring everywhere. Bigger wiring puts a lot less stress on the alternator than trying to shove all that power through little power wires.

TOM HAS SPOKEN.! Thanks.. My system isnt huge so i dont think i have to do a big upgrade to the charging system. Ill hook it all up within the next couple days and check my Voltage drop and go from there... I'll let ya know

You can also look into the alternator sensing wire mod. Dan has a writeup for it here, or ZZP sells harnesses that do the same. I'll be watching for the results!

Hmmm thats neat.

what exactly is that mod? i could have sworn the alt already read battery voltage and adjusted it'* duty cycle based on the voltage...