I have a 1984 F350 crew cab with a 460. I want to add another battery in the engine compartment. I have a spot for it but does anyone know the best way to wire it to run off my current alternator? Any help would great.

 

What are you going to use it for? That will determine how to wire it.

 

I am planning on using it as a secondary battery to run off road lights, Winch.

 

The simplest way is to just hook the two batteries up in parallel, run a big red cable between the positive terminals and ground both negative terminals. This probably isn't the hot ticket for you because it doesn't isolate the two batteries, so if you kill your battery running your winch you won't be able to start your engine. Also it can supposedly cause the two batteries to drain each other when the trucks not running (although I've heard people say that won't happen, so I don't know if that's a real concern).

Another common way is to use a solid state isolator. One cable from the alternator to the isolator, another from the isolator to the main battery and a third cable from the isolator to the secondary battery. This way the two batteries will never take power from each other, and whatever is hooked to one battery will never draw down the other. But the one alternator will still charge both. Downsides to this are the cost of the isolator, isolators can get hot and have started fires (maybe only if something else was really wrong?), and it's essentially a couple of diodes and there's a 0.7 volt drop across a diode, so you'll have slightly lower voltage to everything with the engine running and it'll take slightly longer to charge batteries.

The third way is to use a relay as an isolator. This completely isolates the batteries when the relay is turned off and connects them in parallel when it's turned on. Downsides are cost (not sure how it compares to solid state) and any time the relay is on your heavy-draw items will be pulling down both batteries. But if you wire it to only go on when the engine is running that's not such a big problem. This is the method I used when I put dual batteries on a '95 F-150 (took the setup off when I sold the truck and plan to put it back on one of my trucks someday)

 

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