I'm sure Fordum will chime in here and tell you how to wire your battery with a 10 gauge extension cord and a relay or some diodes, though.
Actually it would take more than diodes and a relay, but I could do it.....
And using an extension cord is not out of the question.
It depends on the cord.
I once helped run an extension cord to a 3 phase machine that used 4 3/0 cables.
Now
that would have made a good battery cable.
But, the key is the copper. The longer the cable, the more copper you got to have.
A short ground cable from the battery to the frame can be as small as 4 gauge and work fine.
I use heaver, but I have see that in factory installs.
How heavy you need to go depends on the length of the wire, not how far the battery is moved.
I had a 14 gauge 50 foot extension cord on a spool that I used for low current setups.
Like moving the receiver and a small TV out to the satellite dish so I could watch while I adjusted it.
One of my friends borrowed it last year at the camp without my knowing it. Next morning they complained about that terrible smell that made them cough all night.
I went in to see what was wrong, and smelled burning insulation instantly. The idiots had plugged in a 1500 watt electric heater into my extension cord. Most of it on the spool had gone from orange to brown.
Their explanation? The heater was only about 2 feet from reaching the outlet, so they figured the extension cord was big enough for that short distance........
So, figure in your cable routing when you work out how far away the battery is.
The insulation is not as important in 12 volt stuff as higher voltages. Most any insulation will keep the electrons in place with no more pressure than that. But, it needs to be rated for the environment.
It is very likely to be exposed to oil, gas, heat, sunlight, vibration, and wear. So it needs to be rated for automotive use.
There is nonautomotive insulation that would work fine if not better than what you get in a parts house, but unless you look up the specs on it, you have no way of knowing.
And, it needs a little extra protection if its going where no battery cable has gone before.
Most battery cables are routed so they do not touch much metal, and are clamped so they don't rub on them when they do.
This cable may run along the frame, through the body, all potential places to wear through, short out, and burn up the truck.
I have advocated running it through PVC pipe or conduit, and have done so when hooking up electric winches on the back end, or a lift gate.
Others here have raised issues with heat melting the PVC, or dirt and water staying in.
All valid points.
You pays your money and you takes your chance.......
Just figure the heaviest copper you think you will need, and consider going up one size.
When the engine is at 70 degrees, its a nice summer day, and the battery has a hot charge that one size extra is not worth the money.
But, when the engine is below zero, the battery is almost dead, and you
really need for it to crank, it can be the difference between walking out and driving.
