Chassis battery ground connections should goto the engine block, not the battery negative terminal

[RF_Burns]

I see a number of references on here showing chassis battery connections (battery charge connection to your trailer battery, DC-DC chargers etc) having the ground connection directly to the negative terminal of the chassis battery. I'm going to make the argument that it should goto the engine block, NOT the chassis battery negative terminal. There should be only one connection to the battery and that is the main ground wire from the battery negative terminal to the engine block.

I spent over 40 years in mobile radio communications systems (Motorola Dealer... Police and Fire Department radios). I've seen lots of burnt wires and damaged equipment from doing this. Side terminal batteries and batteries with a post and stud connection invite issues the most.

Consider this common scenario: You have a large pair of wires run direct from your chassis battery to your new DC-DC charger to charge your new LFP batteries. Normally the chassis engine starter current flows from the positive (we will talk in conventional current flow instead of electron flow) of battery, through the large cable to the starter, which is grounded to the engine block, and back the large ground wire from the engine block to the battery negative terminal. The large wires are designed to carry the hundreds of Amps a starter consumes. The engine starts and all is well.
->But one day... the large negative ground wire from the engine block to the battery gets a bad connection (corrosion, loose terminal etc). Now you try to start the engine and the power flows through the starter to the engine block, but the engine ground is poor, so the current looks to flow in another direction back to ground. The engine block is grounded to the body and frame by smaller ground wires so it finds its way to the House battery negative terminal and back along your DC-DC charger feed supply wires to the chassis battery negative terminal. BUT, none of this wiring was designed to carry the hundred's of Amps a engine starter takes... so things get hot, even burnt and the magic smoke gets out.

Now IF you had the chassis ground wire from your DC-DC charger connected to the engine block, the current would have no where to flow back to the battery as that single wire connection to the battery negative is poor. You would hear a click from the starter solenoid, or maybe not even that. The DC-DC converter would see its power go to near zero volts when you tried the starter, so it would not be harmed.

I know this is a simplification of what would/could and does happen.

So this is my experience and my advice is to never run a ground wire to the negative terminal of your chassis battery. The engine block is the Mother of all Grounds in a vehicle.

So... Only one connection from the chassis battery negative terminal and that is to the engine block.

 

[alfaeric]

Having seen the result of the block randomly choosing a ground path to a chassis, I would argue that you MUST have a strong ground strap between the engine block and the chassis.

 

[RF_Burns]

Having seen the result of the block randomly choosing a ground path to a chassis, I would argue that you MUST have a strong ground strap between the engine block and the chassis.

And also to the body of the vehicle. Generally the engine and drivetrain sit on rubber mounts to the frame. The body sits on rubber mounts to the frame. Both the frame and the body need to be well grounded to the engine block.

 

[alfaeric]

And also to the body of the vehicle. Generally the engine and drivetrain sit on rubber mounts to the frame. The body sits on rubber mounts to the frame. Both the frame and the body need to be well grounded to the engine block.

That applies to trucks, where the body and frame are not the same, but most cars and SUVs, the body and chassis are the same thing.

Some cars do have the powertrain mounted to a subframe, but all OEM's are very aware that floating the ground between the sub-chassis and the rest of the vehicle is a terrible thing.

edit- one other thing- many of the vehicles talked about in this area are trailers. So the chassis ground is the ground, as there's no engine. For my current project- it's a trailer. When moving, the chassis is wired to the car chassis via the 7 pin connector.

 

[RF_Burns]

That applies to trucks, where the body and frame are not the same, but most cars and SUVs, the body and chassis are the same thing.

Some cars do have the powertrain mounted to a subframe, but all OEM's are very aware that floating the ground between the sub-chassis and the rest of the vehicle is a terrible thing.

edit- one other thing- many of the vehicles talked about in this area are trailers. So the chassis ground is the ground, as there's no engine. For my current project- it's a trailer. When moving, the chassis is wired to the car chassis via the 7 pin connector.

The ground wire on the 7-pin connector is a #10 wire in most cases I believe. So running your 50Amp DC-DC charger through it as a power ground is not a good idea. I've seen people using Anderson Power Poles with both positive and negative lines from the tow vehicle to charge the trailer batteries. Especially big 5th wheels with lots of house batteries.
Motorhomes usually have large house battery systems and are built with body on frame construction.

 

[alfaeric]

The ground wire on the 7-pin connector is a #10 wire in most cases I believe. So running your 50Amp DC-DC charger through it as a power ground is not a good idea. I've seen people using Anderson Power Poles with both positive and negative lines from the tow vehicle to charge the trailer batteries. Especially big 5th wheels with lots of house batteries.
Motorhomes usually have large house battery systems and are built with body on frame construction.

What is your alternative?

The ground is chassis ground both in the vehicle and in the trailer. And the chassis ground is grounded both to the battery and the engine. Which means that all grounds are at the same level. As long as the grounds are all common and strong, and rated for the current you are moving- what's the issue?

 

[RF_Burns]

There is no issue IF all the grounds are bonded well and rated for the current they are carrying. BUT, then you install a couple 100AmpHr LiFePO4 batteries with a 50 or 60 Amp DC-DC charger, or Battle-Born's duty-cycle charger system. Now you could be pulling up to 75Amps from the tow vehicle and your 7 pin connector cannot handle that, plus lights and electric brakes as well.
In that case I would pull a separate ground wire (cable) from the engine block to the ground connection of your batteries. You will also need a separate positive cable fused at the chassis battery terminal. The cable size will need to be determined by the current the batteries (charger system) will consume and the length of the cables. You will also need to use a water-proof connector at the hitch of sufficient current rating so you can disconnect.
My Motorhome has a 0 gauge wire from the chassis battery location back to the house batteries and DC distribution panel. The two batteries are isolated to prevent common discharge.

 

[alfaeric]

For that, the trailer lights are not powered by the 12V power supply in the 7 pin connector- they are fed on their own circuit- at least in my case. And it should be in every case- that should be totally driven off of the vehicle's light system for the brake/turn signal and running lights.

But your greater point is to make sure that everything is wired, grounded, and fused properly. As long as that is done, it's not required to run a 00 welding wire from your trailer battery to your vehicle battery. Make sure the IF is in fact true.