I am surprised nobody else said this, but you need more than just another battery.
The formula is this: amps X volts = watts that means if you want 7,000 watts at 14.4 volts, you will be sucking 486 amps at 100% efficiency. You won't be 100% efficient so to be safe you will need at least 500 amps worth of charging power to break even. That means you need a second alternator or you will run your battery bank dry very quickly and have horrendous voltage drop.
Voltage drop makes the problem WORSE! Figure you want 7kw but have voltage drop down to 12v. That means you need 583 amps at 100% efficiency. Just food for thought.
No matter what though, you'll need at least two more batteries. Those amps should have AT LEAST two batteries each. Two runs of 1/0 for power and two runs of 1/0 for ground should be plenty. Don't bother with a chassis ground though, run direct from under hood to your battery bank.
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