DC-DC Chassis ground question

[benoit.letourneau@panomed]

Hi

Out of curiosity, are the two diagrams below equivalent electrically speaking. I mean for the chassis ground. I want to use the chassis for the negative return instead of running neg wires back to each source.

Thanks

 

[Supervstech]

It depends on the distance to the voltage source, and the load on the battery...

Always more efficient to run rated copper to the loads...

 

[Supervstech]

Keep in mind, all the loads in an auto that rely on the chassis for negative are usually LOW demand items like lights...
The starter works because it is directly attached to the main engine block where the main negatives all connect directly, and the alternator body is directly connected to the block...

So, unless your intended loads are small or directly connected to the block... I'd run copper the correct ampacity.

 

[benoit.letourneau@panomed]

Keep in mind, all the loads in an auto that rely on the chassis for negative are usually LOW demand items like lights...
The starter works because it is directly attached to the main engine block where the main negatives all connect directly, and the alternator body is directly connected to the block...

So, unless your intended loads are small or directly connected to the block... I'd run copper the correct ampacity.

What would you consider big

 

[Supervstech]

What would you consider big

Anything over 20W

 

[benoit.letourneau@panomed]

What would you consider big

I know the battery neg is attached to the chassis with a cable. I will check the size.

 

[mikefitz]

that rely on the chassis for negative are usually LOW demand items like lights.

Most vehicles use the body/ frame/ chassis for negative return current for alternator and starter as well as lighting and accessories. The engine block is connected to frame/ body with a braded strap.
In the instalation of a battery to battery charger in a metal bodied vehicle its possible to use a chassis connection as shown in the diagram. In many modern vehicles there are current sensors in the vehicle battery negative path to chassis. To avoid upsetting the vehicle system , any added battery to battery charger must be connected to chassis, not the starter battery negative post.
For safety the house battery should be connected to body/chassis.
In a situation where its doubtful a ' good' connection to body/chassis can be made, a suitable cable can be run from house battery negative to starter battery negative chassis/body point, ( not negative battery post).

 

[benoit.letourneau@panomed]

Most vehicles use the body/ frame/ chassis for negative return current for alternator and starter as well as lighting and accessories. The engine block is connected to frame/ body with a braded strap.
In the instalation of a battery to battery charger in a metal bodied vehicle its possible to use a chassis connection as shown in the diagram. In many modern vehicles there are current sensors in the vehicle battery negative path to chassis. To avoid upsetting the vehicle system , any added battery to battery charger must be connected to chassis, not the starter battery negative post.
For safety the house battery should be connected to body/chassis.
In a situation where its doubtful a ' good' connection to body/chassis can be made, a suitable cable can be run from house battery negative to starter battery negative chassis/body point, ( not negative battery post).

Ok so it is a good practice to use the chassis?

I did some investigations a found that the the battery neg is connected to the chassis. The wire is probably a awg 2 (much bigger than the 4 I am using). That is the only connection on the negative side. It means everything is using the chassis for neg return.

My alternator is rated for 220 amps. I will use a 40 amp charger which may draw up to 60a. Not sure how much the car is using on its own but I suspect it's under 80 (I read 60, some other place 75) so there is enough margin to run my dc-dc.

I was just wondering if I could put both (input and output) neg to the chassis. My understanding is yes as everything is bond to the chassis anyway as the whole system is "grounded" to the chassis.

Thank you

 

[Supervstech]

Ok so it is a good practice to use the chassis?

I did some investigations a found that the the battery neg is connected to the chassis. The wire is probably a awg 2 (much bigger than the 4 I am using). That is the only connection on the negative side. It means everything is using the chassis for neg return.

My alternator is rated for 220 amps. I will use a 40 amp charger which may draw up to 60a. Not sure how much the car is using on its own but I suspect it's under 80 (I read 60, some other place 75) so there is enough margin to run my dc-dc.

I was just wondering if I could put both (input and output) neg to the chassis. My understanding is yes as everything is bond to the chassis anyway as the whole system is "grounded" to the chassis.

Thank you

Ok, let me try again...

For auto loads, and alternator loads, vehicle lighting etc, chassis connections is fine... done to save wiring costs, and losses won't matter...
BUT any significant or important loads ALWAYS get an actual direct to bussbar connection...

Even in automobiles... the computer, radio, dashboard, climate controls... anything where clean power and steady voltage is needed... gets a dedicated wire for hot and grounded conductor...

Yes, all metal parts of the vehicle have bonding connections... and yes, many loads in the car are connected to the chassis...

But the best cleanest, least loss lowest voltage drop is a wired connection direct to the battery, or off a bussbar connected to LARGE steel bodyparts... as close to the battery as feasible.

Will a chassis only system work? Sure... but do you want to waste the wattage of non copper, oxidized, unknown chassis system?

 

[benoit.letourneau@panomed]

Ok, let me try again...

For auto loads, and alternator loads, vehicle lighting etc, chassis connections is fine... done to save wiring costs, and losses won't matter...
BUT any significant or important loads ALWAYS get an actual direct to bussbar connection...

Even in automobiles... the computer, radio, dashboard, climate controls... anything where clean power and steady voltage is needed... gets a dedicated wire for hot and grounded conductor...

Yes, all metal parts of the vehicle have bonding connections... and yes, many loads in the car are connected to the chassis...

But the best cleanest, least loss lowest voltage drop is a wired connection direct to the battery, or off a bussbar connected to LARGE steel bodyparts... as close to the battery as feasible.

Will a chassis only system work? Sure... but do you want to waste the wattage of non copper, oxidized, unknown chassis syst

Ok I get your point and that is exactly what I did for all my circuit. A red and a black creating a closed loop from my "house" battery. 100% with you on that. Less interference, cleaner design, easier to troubleshoot in case of problems, etc.

What I don't get though, is whether or not I connect my negative (on the input side of my charger) to the starter battery neg post or to the metal of the chassis, it won't really change anything because the current is going from the alternator to the battery using the chassis. I checked and negative of the battery is not connected to anything else but the chassis. Why would I use a cable to the battery when the negative path between the alternator and the battery is already using the chassis. Regardless of where I will connect that black wire, the current will inevitably run through the frame as there is no other path. I have a Ram Promaster and the negative runs to the frame using about 6 or 7 inches of cable. My guess is that is AWG 3 or AWG 2.

I don't want to argue, I just want to understand why the car itself uses a chassis ground at the battery.

 

[Supervstech]

Ok I get your point and that is exactly what I did for all my circuit. A red and a black creating a closed loop from my "house" battery. 100% with you on that. Less interference, cleaner design, easier to troubleshoot in case of problems, etc.

What I don't get though, is whether or not I connect my negative (on the input side of my charger) to the starter battery neg post or to the metal of the chassis, it won't really change anything because the current is going from the alternator to the battery using the chassis. I checked and negative of the battery is not connected to anything else but the chassis. Why would I use a cable to the battery when the negative path between the alternator and the battery is already using the chassis. Regardless of where I will connect that black wire, the current will inevitably run through the frame as there is no other path. I have a Ram Promaster and the negative runs to the frame using about 6 or 7 inches of cable. My guess is that is AWG 3 or AWG 2.

I don't want to argue, I just want to understand why the car itself uses a chassis ground at the battery.

Ahh.
Yeah, for that it doesn't matter. Sure, attach to the chassis.

As to the alternator circuit, obviously the alternator is connected directly to the engine, a massive steel structure that is also bonded to the chassis in several places.
But any electronic devices will have the ground potential wires running directly to the engine.
The place where the alternator is connected.

 

[mikefitz]

a massive steel structure that is also bonded to the chassis in several places.

No you are incorrect. The engine bond to chassis/ vehicle metal is normally a single 'earth strap' of copper blade, some vehicles may have a second strap. All current for engine accessories, starter motor and alternator pass through this and the vehicle metal to the nbattery negative
.

This is the single connection on a RamPromaster, in Europe sold as a Fiat Ducato. The Ford transit has a similar connecting strap.
There is no problem using the metal body of a light van for negative return for a battery to battery charger. The manufacturer uses the metalwork for starter current and alternator charge current negative path.

Mike

 

[mikefitz]

just want to understand why the car itself uses a chassis ground at the battery.

It's more reliable and saves copper. A modern luxury car will have many distributed electronic modules, actuators, sensors, electric motors, all use the metal body for negative return current along with the starter and alternator.

 

[mikefitz]

Even in automobiles... the computer, radio, dashboard, climate controls... anything where clean power and steady voltage is needed... gets a dedicated wire for hot and grounded conductor...

Again no, you are not correct.
As a one time auto electrician and a one time design engineer for engine ECU, I can state that each and every module and electrical unit uses vehicle metal as negative return current to the vehicle battery .
Examine the vehicle battery and identify all the dedicated negatives, you won't find any because they are distributed through the vehicle metal structure.

 

[Supervstech]

No you are incorrect. The engine bond to chassis/ vehicle metal is normally a single 'earth strap' of copper blade, some vehicles may have a second strap. All current for engine accessories, starter motor and alternator pass through this and the vehicle metal to the nbattery negative
.View attachment 170025

This is the single connection on a RamPromaster, in Europe sold as a Fiat Ducato. The Ford transit has a similar connecting strap.
There is no problem using the metal body of a light van for negative return for a battery to battery charger. The manufacturer uses the metalwork for starter current and alternator charge current negative path.

Mike

I’m sorry, but you are misunderstanding how the electrical system is working in the car…
Loads don’t come from the battery… they come from the alternator… except when starting the car.
And the alternator is directly attached to the block.

Follow the source.
Yes, many cars have a clear chassis connection point that all critical loads originate from.

 

[benoit.letourneau@panomed]

Again no, you are not correct.
As a one time auto electrician and a one time design engineer for engine ECU, I can state that each and every module and electrical unit uses vehicle metal as negative return current to the vehicle battery .
Examine the vehicle battery and identify all the dedicated negatives, you won't find any because they are distributed through the vehicle metal structure.

It does make sense. There are many wires leaving the positive post of the battery and only one for the negative and, that negative is connected directly to the car chassis. This mean everything running in the ultimately runs back via the metal frame of the car. That's my understanding...

 

[benoit.letourneau@panomed]

Not sure I follow you, I looked around and there are many ground point throughout the vehicle. Yes several connections are made at each point but there are many of them. This leads me to believe the frame is used abundantly

 

[Zwy]

Ok so it is a good practice to use the chassis?

No

I did some investigations a found that the the battery neg is connected to the chassis. The wire is probably a awg 2 (much bigger than the 4 I am using). That is the only connection on the negative side. It means everything is using the chassis for neg return.

My alternator is rated for 220 amps. I will use a 40 amp charger which may draw up to 60a. Not sure how much the car is using on its own but I suspect it's under 80 (I read 60, some other place 75) so there is enough margin to run my dc-dc.

I was just wondering if I could put both (input and output) neg to the chassis. My understanding is yes as everything is bond to the chassis anyway as the whole system is "grounded" to the chassis.

Thank you

Do yourself and your system a favor. Run a negative cable instead of using a "chassis ground" point.

Having to troubleshoot DC circuits for a living will make you understand why a negative lead is always preferred to a "chassis ground" for any electrical load. The main reason automotive manufacturers use the "chassis ground" point is to save on costs to manufacture a product.

I could elaborate but it isn't worth the time. If you want to use a "chassis ground" point, then do so. Be aware it does have some drawbacks.

 

[Zwy]

Again no, you are not correct.
As a one time auto electrician and a one time design engineer for engine ECU, I can state that each and every module and electrical unit uses vehicle metal as negative return current to the vehicle battery .
Examine the vehicle battery and identify all the dedicated negatives, you won't find any because they are distributed through the vehicle metal structure.

While you may have been a "one time auto electrician", you failed to realize sensors will normally have a negative lead back to source, this is to provide 0V potential. I can remember the famous GM ground point on the engines of the 90's where the thermostat housing bolt was the ground point. Certainly lead to many issues. Another is the ground point under the cab to the chassis on model year 2000 and up GM trucks that was famous for corrosion leading to false codes in the ABS system, gauge problems and any other module attached to the great spot the idiot decided was a good idea.

Just because one can, does not always make it a wise decision.