we are putting dual batteries onto our DIY livingquarter. the inside already has an inverter. we can already run off a generator. but we are looking to put batteries in so that we can have enough pover to run lights or small thinngs without turning on generator.

So I have a battery box and main shot off switch/ diconnect. but I know there is a part that only lets the current go one direction, but dont know the name or where to find it?

I believe you're looking for a battery isolater.  There are other technologies besides the diode.

An "integrator" uses a contact so there isn't a voltage drop.

click here for more info ...  http://www.newmarpower.com/Battery_Isolator_Integrators/Battery_Isolator_Integrators.html