As long as the ground potential is the same you will be OK. I'm sure some of the electrical engineers (FORDUM) will jump in with the exact science of it all.
I'll dip in a toe rather than jump all the way in.
Under ideal circumstances, as long as there is no difference in voltage potential between the grounds you are fine. Even with a small amount you should not have too much trouble.
Which means if everything is bonded together by being bolted together solidly or with bonding wires or straps, then there should be no problem with running one wire to the body and one to the engine block.
But, if everything were perfect, we would not have a garage forum, because we would never be working on our trucks.
When designing wiring systems, you never figure on best case, always worst case.
If there is no short, you don't need that fuse.
Most fuses are a huge waste of money.
They just sit there and do nothing.
Except when they are needed.
Then they can be worth more than gold.
Gold is about $1200 per ounce. How much is it worth for your truck not to burn up?
Hooking up battery cables is a lot like that. Every time I have hooked up dual batteries, I always make sure that the ground cables are both hooked to the same continuous piece of metal.
Both to the engine block is my usual place. But, both to the frame is not bad. But, I would never go one to the block and one to the body.
If you lost a ground strap, you might get all kinds of voltages in strange places. Under certain circumstances, you could wind up with 24 volts on some components.
And even if going to the block, I would make sure I went both to the block and not one to the block and one to a bracket mounted on the block.
The neatest hookups I remember, used a set of cables off a diesel.
One of them, had a regular top post battery clamp which had a battery post molded into the clamp. The second battery used a double ended battery cable to hook them together.
And the other one, used marine type clamps with the stainless steel stud and nut on top of the clamp. Both battery cables were super heavy ring terminals.
However you do it, BE CAREFUL of that second positive wire.
When you run it from one battery to the other, think long term.
It usually goes across the engine compartment, which means it goes past or over all kinds of sharp or hot things.
Wire strapping it to that thin edged radiator support is fine today, but what happens after 20 or 30 thousand miles of pounding on dirt roads and vibration?
The only thing that saved a friend's truck was that the thin edge that wore through the insulation was too thin to stand up to the current. It melted and blew out before the wire could set anything else on fire. He saw the flash at night and opened the hood to see the smoke.
All my double battery setups have the positive cable run inside a piece of gray PVC conduit. I put the cable in, and gently heat the pipe to bend it to conform to what it has to travel around.
Dang, I only wanted to put my toe in, but got both feet wet...
J.
