3rd Battery?
 

Just acquired a 3rd battery. Its not made with vehicles in mind. Its rated for 100amps/8hrs. It is used to run cell phone towers in case of power outrage. But I have heard of one being used to jump a car. They are 12v. How would this work with stock alternator and starter? I'm going to make it to where I can isolate the batteries from the other 2 so I can run an inverter all night off that battery without worrying about drawing the other batteries.

I would just hook it up via isolator and run your accessories off of it. The isolator only allows the battery to be re-charged and does no allow current to flow the other way, basically it will not have any effect on your starting system.

Another option would be to run a heavy duty solenoid that activates when the truck is running via oil pressure switch or something similar to keep the battery charged, then wire a momentary switch inside the cab just in case you need a little extra boost for starting.

Thanks 907dave. I like the isolator idea...seems less troublesome and worry free. I may add a relay later to give juiceg back to the batteries for a boost.

Keep in mind that battery probably wasn't designed for the vibration and abuse riding in a truck will give it. Hope it holds up though cause it should run inverter for days. :D

Yea, I have nothing invested and he comes around about every 2yrs.

Can someone post a product page of an isolator. Everything I'm finding has something to do with filtering noise out of the current for audio devices.