Adding 1000 rms. What gauge wire?

[IDSkoT 5,000+ posts]

Don't use 1 gauge, use 1 ought ... 1awg and 1/0 is a bit (big) of a difference.
If you were to use the scrap 4awg to power up your 2nd battery, what you have to keep in mind, is that when you parrallel runs of power / ground to ANYTHING, they have to be EXACTLY the same length (And I'm talking to the 1/4 inch or less) otherwise, electricity will follow the path of least resistance, and just blow up the shortest run of 4awg, then start to use the longer one as it's only path (If your car hasn't burnt to a crisp).

I'd suggest using 3 runs of + and 3 runs of - all EXACTLY the same length of the 4awg if your set on going that route, but a single run of 1/0 is much better idea

No it's not.

1/0 AWG = 0 AWG

Last time I checked, 1 AWG and 0 AWG are close in size...

0 AWG can handle about 50 or 60 amps more. But that's usual as sizes go up.

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[alvitae 10+ year member]

two runs of 4ga is equal to one 1ga. you will be fine with double runs of 4ga between the batt(s) and alt., especially at such short lengths. make sure to keep those wires neat and away from moving parts/hot areas. when you start adding batteries and more and more wire things can get risky under the hood.

x2