Dual Batteries

[wildcard]

I'm thinking of adding a 2nd battery under the hood to attach an inverter to. Got a 300 I6 with just a stock alternator.
What would be the process for hooking them parallel. I know it's + to + and - to -, but do I need a cut-off switch between them, just switching it on when the 2nd battery needs a charge? And can I use my existing alternator to charge both batteries?
Thanks

[HIO Silver]

As proposed and electrically speaking, it'll act as if it only has one battery with twice the reserve capacity.

The correct way to have dual batteries is with a battery isolator which separates the main battery from the auxiliary battery and a battery selector switch (off, on, and parallel). In a pinch, it will also allow you to self-jump should the main battery go dead.

Here's some required reading:
Dual Battery Systems
4Crawler Cheap Trick: Dual Batteries
FSB Thread: Dual Batteries
FTE Thread: Dual Battery Installation By NBC_Steve
Hellroaring Battery Isolators

[wildcard]

HIO Silver, that was excellent info. Thanks a lot.
I think I want to add the "high current relay". Is this the same thing as a regular starter solenoid? If not, what is the official name so I know what to ask for. My understanding is the cable runs from the battery side of the starter solenoid to the relay and out the other side of the relay to the aux. battery. Correct?

[HIO Silver]

You,ll need a continuous duty solenoid. A starter relay works on the same principle but the internals are not designed for prolonged use.

Follow the wiring diagrams and you'll be on your way.

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