When not to neglect the resistance of a car body

labora

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Using high current devices combined with currents through the body made me think. So I concluded with a practical approach where a chassis contact is made for every 50 A with a minimum distance of 100 mm. I could not find any information on the subject so I decided to create a model. Apparently some people hate links on this forum, but I don't want to paste it all here. I would really appreciate some review. See https://vanderworp.org/electrical-resistance-of-the-car-body/.
 

HaldorEE

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Deliberately using the chassis for high current? That reminds me of the guy from "Down Periscope."


I prefer to use a separate conductor to carry return current. I am not responsible for what Ford did in the engine compartment, but in my RV's house DC wiring, the negative bus only connects to chassis ground at a single point.
 
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labora

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Deliberately using the chassis for high current? That reminds me of the guy from "Down Periscope."
What a beautiful metaphor! LOL
I prefer to use a separate conductor to carry return current. I am not responsible for what Ford did in the engine compartment, but in my RV's house DC wiring, the negative bus only connects to chassis ground at a single point.
Okay, that is what my mentor said to me too.

Thanks for taking the time to study the model. Here is what I wrote after editing...


Conclusions​

  • In situations with high currents:
    • Never neglect the resistance of a car’s bodywork.
    • If you absolutely must use the bodywork, you must have several connections to the bodywork with sufficient space between the connections.
    • If possible, do not use the bodywork at all for (high) currents.
      • Do not confuse earthing with the use of the bodywork as an additional conductor.
      • Consider poor connections due to corrosion of bare mild steel over time.
      • Consider galvanic effects on the bodywork as a whole.
      • Consider the possibility of enormous heat development at such grounding points.

Some important personal notes​

  • I would probably create a set of grounding points…
    • One contact point for each 50 A
    • Distanced at 100 mm at least
    • Sized M6 or M8, properly torqued
    • Localized on a beam or similar, with enough material thickness.
    • Combined, interconnected, as the sole earthing point.
    • The method of mounting bolts and washers is of course very critical in this setup.
 

amaclach

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Also remember that steel is not a great conductor - nowhere near as good as copper wire, you will see losses using the steel bodywork for the return path.

Electrical conductivity in metals is a result of the movement of electrically charged particles. The atoms of metal elements are characterized by the presence of valence electrons, which are electrons in the outer shell of an atom that are free to move about. It is these "free electrons" that allow metals to conduct an electric current.
Because valence electrons are free to move, they can travel through the lattice that forms the physical structure of a metal. Under an electric field, free electrons move through the metal much like billiard balls knocking against each other, passing an electric charge as they move.

Transfer of Energy
The transfer of energy is strongest when there is little resistance. On a billiard table, this occurs when a ball strikes against another single ball, passing most of its energy onto the next ball. If a single ball strikes multiple other balls, each of those will carry only a fraction of the energy.
By the same token, the most effective conductors of electricity are metals that have a single valence electron that is free to move and causes a strong repelling reaction in other electrons. This is the case in the most conductive metals, such as silver, gold, and copper. Each has a single valence electron that moves with little resistance and causes a strong repelling reaction.
Semiconductor metals (or metalloids) have a higher number of valence electrons (usually four or more). So, although they can conduct electricity, they are inefficient at the task. However, when heated or doped with other elements, semiconductors like silicon and germanium can become extremely efficient conductors of electricity.

Metal Conductivity​

Conduction in metals must follow Ohm's Law, which states that the current is directly proportional to the electric field applied to the metal. The law, named after German physicist Georg Ohm, appeared in 1827 in a published paper laying out how current and voltage are measured via electrical circuits. The key variable in applying Ohm's Law is a metal's resistivity.
Resistivity is the opposite of electrical conductivity, evaluating how strongly a metal opposes the flow of electric current. This is commonly measured across the opposite faces of a one-meter cube of material and described as an ohm meter (Ω⋅m). Resistivity is often represented by the Greek letter rho (ρ).
Electrical conductivity, on the other hand, is commonly measured by siemens per meter (S⋅m−1) and represented by the Greek letter sigma (σ). One siemens is equal to the reciprocal of one ohm.

Conductivity, Resistivity of Metals​

Material​

Resistivity
p(Ω•m) at 20°C​

Conductivity
σ(S/m) at 20°C​

Silver1.59x10-86.30x107
Copper1.68x10-85.98x107
Annealed Copper1.72x10-85.80x107
Gold2.44x10-84.52x107
Aluminum2.82x10-83.5x107
Calcium3.36x10-82.82x107
Beryllium4.00x10-82.500x107
Rhodium4.49x10-82.23x107
Magnesium4.66x10-82.15x107
Molybdenum5.225x10-81.914x107
Iridium5.289x10-81.891x107
Tungsten5.49x10-81.82x107
Zinc5.945x10-81.682x107
Cobalt6.25x10-81.60x107
Cadmium6.84x10-81.467
Nickel (electrolytic)6.84x10-81.46x107
Ruthenium7.595x10-81.31x107
Lithium8.54x10-81.17x107
Iron9.58x10-81.04x107
Platinum1.06x10-79.44x106
Palladium1.08x10-79.28x106
Tin1.15x10-78.7x106
Selenium1.197x10-78.35x106
Tantalum1.24x10-78.06x106
Niobium1.31x10-77.66x106
Steel (Cast)1.61x10-76.21x106
Chromium1.96x10-75.10x106
Lead2.05x10-74.87x106
Vanadium2.61x10-73.83x106
Uranium2.87x10-73.48x106
Antimony*3.92x10-72.55x106
Zirconium4.105x10-72.44x106
Titanium5.56x10-71.798x106
Mercury9.58x10-71.044x106
Germanium*4.6x10-12.17
Silicon*6.40x1021.56x10-3

*Note: The resistivity of semiconductors (metalloids) is heavily dependent on the presence of impurities in the material.
Bell, Terence. (2020, October 29). Electrical Conductivity of Metals. Retrieved from https://www.thoughtco.com/electrical-conductivity-in-metals-2340117
 
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labora

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Also remember that steel is not a great conductor - nowhere near as good as copper wire, you will see losses using the steel bodywork for the return path.
Mild steel is not in the table. The value I used for the body model is 15*10^-8 Ohm.m. Some safety margins should be used.

Steel alloyed with chrome and nickel has a significantly higher resistance. This is a pleasant property when TIG welding.

Copper, pure and electrolytically produced, has the properties shown in the table. Despite the high purity of copper for water pipes (99.9%), the small contamination with phosphorus is responsible for a significant increase in resistance. I calculate with 20%.

For the general calculation model of the bodywork, this value is sufficient, i.e. more qualitative.

Do you also have an opinion on the model itself? As indicated in the link?

By the way, a nice table and a pleasant readable explanation.
 

amaclach

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Mild steel is not in the table. The value I used for the body model is 15*10^-8 Ohm.m. Some safety margins should be used.

Steel alloyed with chrome and nickel has a significantly higher resistance. This is a pleasant property when TIG welding.

Copper, pure and electrolytically produced, has the properties shown in the table. Despite the high purity of copper for water pipes (99.9%), the small contamination with phosphorus is responsible for a significant increase in resistance. I calculate with 20%.

For the general calculation model of the bodywork, this value is sufficient, i.e. more qualitative.

Do you also have an opinion on the model itself? As indicated in the link?

By the way, a nice table and a pleasant readable explanation.
I have no issue with your work, I'm simply stating that the return path for high current applications would be preferable via an appropriately sized copper conductor rather than the vehicle's steel body or chassis. I think we all agree on that.
 

labora

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There is indeed agreement. The reason for making this post was that it is nice to spar about the model itself and to pick out errors. In a way, that has been done. I have one chance to build it right the first time. The model provides quite a few indications from a theoretical view to conclude that it is better to use separate conductors - despite the often daily practice at garages. Thanks!
 

ultrakiev

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Messages
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Also remember that steel is not a great conductor - nowhere near as good as copper wire, you will see losses using the steel bodywork for the return path.

Electrical conductivity in metals is a result of the movement of electrically charged particles. The atoms of metal elements are characterized by the presence of valence electrons, which are electrons in the outer shell of an atom that are free to move about. It is these "free electrons" that allow metals to conduct an electric current.
Because valence electrons are free to move, they can travel through the lattice that forms the physical structure of a metal. Under an electric field, free electrons move through the metal much like billiard balls knocking against each other, passing an electric charge as they move.

Transfer of Energy
The transfer of energy is strongest when there is little resistance. On a billiard table, this occurs when a ball strikes against another single ball, passing most of its energy onto the next ball. If a single ball strikes multiple other balls, each of those will carry only a fraction of the energy.
By the same token, the most effective conductors of electricity are metals that have a single valence electron that is free to move and causes a strong repelling reaction in other electrons. This is the case in the most conductive metals, such as silver, gold, and copper. Each has a single valence electron that moves with little resistance and causes a strong repelling reaction.
Semiconductor metals (or metalloids) have a higher number of valence electrons (usually four or more). So, although they can conduct electricity, they are inefficient at the task. However, when heated or doped with other elements, semiconductors like silicon and germanium can become extremely efficient conductors of electricity.

Metal Conductivity​

Conduction in metals must follow Ohm's Law, which states that the current is directly proportional to the electric field applied to the metal. The law, named after German physicist Georg Ohm, appeared in 1827 in a published paper laying out how current and voltage are measured via electrical circuits. The key variable in applying Ohm's Law is a metal's resistivity.
Resistivity is the opposite of electrical conductivity, evaluating how strongly a metal opposes the flow of electric current. This is commonly measured across the opposite faces of a one-meter cube of material and described as an ohm meter (Ω⋅m). Resistivity is often represented by the Greek letter rho (ρ).
Electrical conductivity, on the other hand, is commonly measured by siemens per meter (S⋅m−1) and represented by the Greek letter sigma (σ). One siemens is equal to the reciprocal of one ohm.

Conductivity, Resistivity of Metals​

Material​

Resistivity
p(Ω•m) at 20°C​

Conductivity
σ(S/m) at 20°C​

Silver1.59x10-86.30x107
Copper1.68x10-85.98x107
Annealed Copper1.72x10-85.80x107
Gold2.44x10-84.52x107
Aluminum2.82x10-83.5x107
Calcium3.36x10-82.82x107
Beryllium4.00x10-82.500x107
Rhodium4.49x10-82.23x107
Magnesium4.66x10-82.15x107
Molybdenum5.225x10-81.914x107
Iridium5.289x10-81.891x107
Tungsten5.49x10-81.82x107
Zinc5.945x10-81.682x107
Cobalt6.25x10-81.60x107
Cadmium6.84x10-81.467
Nickel (electrolytic)6.84x10-81.46x107
Ruthenium7.595x10-81.31x107
Lithium8.54x10-81.17x107
Iron9.58x10-81.04x107
Platinum1.06x10-79.44x106
Palladium1.08x10-79.28x106
Tin1.15x10-78.7x106
Selenium1.197x10-78.35x106
Tantalum1.24x10-78.06x106
Niobium1.31x10-77.66x106
Steel (Cast)1.61x10-76.21x106
Chromium1.96x10-75.10x106
Lead2.05x10-74.87x106
Vanadium2.61x10-73.83x106
Uranium2.87x10-73.48x106
Antimony*3.92x10-72.55x106
Zirconium4.105x10-72.44x106
Titanium5.56x10-71.798x106
Mercury9.58x10-71.044x106
Germanium*4.6x10-12.17
Silicon*6.40x1021.56x10-3

*Note: The resistivity of semiconductors (metalloids) is heavily dependent on the presence of impurities in the material.
Bell, Terence. (2020, October 29). Electrical Conductivity of Metals. Retrieved from https://www.thoughtco.com/electrical-conductivity-in-metals-2340117

There is indeed agreement. The reason for making this post was that it is nice to spar about the model itself and to pick out errors. In a way, that has been done. I have one chance to build it right the first time. The model provides quite a few indications from a theoretical view to conclude that it is better to use separate conductors - despite the often daily practice at garages. Thanks!
Do you think I should have a problem running 20A 12v load thru SUV body on 2 meters distance between the battery negative terminal and the ground tap for my load? I tested it it's working now. Just wondering what long-term issues I could have except corrosion?
 

labora

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Do you think I should have a problem running 20A 12v load thru SUV body on 2 meters distance between the battery negative terminal and the ground tap for my load? I tested it it's working now. Just wondering what long-term issues I could have except corrosion?
I do not think that is wrong as in "right or wrong". For many years I enjoyed an inverter connected like this, without a household battery. Whenever I drew power, around 50 A, it was always with the engine running. Last year the starter battery was worn out... After 12 years. Proper earthing points with thread are welded in from the factory. So the only responsibility we have is to make good contact with a bolt and washers tightened to the appropriate torque. Perhaps others think differently.
 

amaclach

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Messages
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I do not think that is wrong as in "right or wrong". For many years I enjoyed an inverter connected like this, without a household battery. Whenever I drew power, around 50 A, it was always with the engine running. Last year the starter battery was worn out... After 12 years. Proper earthing points with thread are welded in from the factory. So the only responsibility we have is to make good contact with a bolt and washers tightened to the appropriate torque. Perhaps others think differently.
Agreed.
It it ain't broke, don't fix it.
The only benefit that you would get would be fewer losses, but if you are happy right now, then you should be good to go.

I have a 12v compressor mounted in my engine bay. It draws about 30A. The manufacturer recommends NOT to earth it to the body. it is mounted on the opposite fender to the battery, so the cable run is <2m. In the interest of science, I tried it earthed to the body and it worked. I then tried using the manufacturer's recommendation of using a copper wire (supplied with the compressor) back to the battery - It worked better - noticeably better.
Having said that and knowing that, I have a DC-DC charger in the back of my vehicle to charge the house lithium setup. It will draw what it needs in order to provide ~350w to the house batteries. I have that earthed to the body of the vehicle because I didn't want to run another thick cable. What that means is, because of the added resistance, I'll be getting a voltage drop, which means more current, which means more load on the alternator. Everything is within spec, but it could be better. If I was drawing more than the 25A (ish) that I am now, then I would definitely be looking at a return cable.
 

HaldorEE

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There is indeed agreement. The reason for making this post was that it is nice to spar about the model itself and to pick out errors. In a way, that has been done. I have one chance to build it right the first time. The model provides quite a few indications from a theoretical view to conclude that it is better to use separate conductors - despite the often daily practice at garages. Thanks!
Good luck man.

Is this for an MS?
 

Batvette

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I used to do car audio competition, and this was always a highly debated subject.
1. When you say through the BODY, you want the conductor to be the unibody or the frame, not the body pieces of a body on frane vehicle.
2. A fundamental people get wrong is that the ground point of your circuit is NOT the battery negative terminal. Its the frame/unibody of the vehicle. Repeat as often as needed because...
3. Youve got a safety issue if you hook up any accessory to a vehicle and use an additional cable to run ground back to the starting battery. If the OEM neg cable fails when you crank the starter, its full current goes through that accessory. Fire, smoke, ruined component.
Really, just never forget #2. The common ground of our circuit is not battery negative, its vehicle chassis. With that in mind the conclusion is its pointless to run additional negative cables from components to power supplies. #3 tells us: Redundancy is not insurance, its liability.
 
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labora

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Good luck man.

Is this for an MS?
Certainly, my girlfriend is also welcome on board my NRV (Non Recreational Vehicle). But seriously, I have the anomaly of sometimes delving deep into things because I want to understand them and do something with them in a responsible way. I have mentors but I am certainly not busy with school as a 59-year-old. With Labora, I am trying to achieve a non-orthodox NRV. The starting points for Labora can be found here.
 

labora

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3. Youve got a safety issue if you hook up any accessory to a vehicle and use an additional cable to run ground back to the starting battery. If the OEM neg cable fails when you crank the starter, its full current goes through that accessory.
I am trying to understand. What am I missing here? Could you elaborate a bit more on that? This is probably not what you wanted to state:
IMG_20210324_011337_.png
 

Batvette

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I am trying to understand. What am I missing here? Could you elaborate a bitev more on that? This is probably not what you wanted to state:
Should the oem chassis to battery negative fail, the only path for the starter becomes the negative cable you added. It will get there by passing through your accessory, which is usually mounted to metal. Thats the flaw in your schematic is it shows the accessory in isolation to chassis ground. The path from chassis ground to negative terminal in the accessory generally goes through its internal circuitry, thats whats gonna burn.
 

Batvette

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Agreed.
It it ain't broke, don't fix it.
The only benefit that you would get would be fewer losses, but if you are happy right now, then you should be good to go.

I have a 12v compressor mounted in my engine bay. It draws about 30A. The manufacturer recommends NOT to earth it to the body. it is mounted on the opposite fender to the battery, so the cable run is <2m. In the interest of science, I tried it earthed to the body and it worked. I then tried using the manufacturer's recommendation of using a copper wire (supplied with the compressor) back to the battery - It worked better - noticeably better.
Having said that and knowing that, I have a DC-DC charger in the back of my vehicle to charge the house lithium setup. It will draw what it needs in order to provide ~350w to the house batteries. I have that earthed to the body of the vehicle because I didn't want to run another thick cable. What that means is, because of the added resistance, I'll be getting a voltage drop, which means more current, which means more load on the alternator. Everything is within spec, but it could be better. If I was drawing more than the 25A (ish) that I am now, then I would definitely be looking at a return cable.
The improvement you saw might have been equally realized by augmenting or tightening/cleaning the existing chassis to battery negative cables.

In these discussions you have to ask yourself how many negative wires do you see scattered around the car going back to the battery? How about none? The mfr knows what they are doing.
If you add accessories with a high current load its worth augmenting the battrery neg to chassis cable but thats about it.
 

HaldorEE

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Should the oem chassis to battery negative fail, the only path for the starter becomes the negative cable you added. It will get there by passing through your accessory, which is usually mounted to metal. Thats the flaw in your schematic is it shows the accessory in isolation to chassis ground. The path from chassis ground to negative terminal in the accessory generally goes through its internal circuitry, thats whats gonna burn.
And that is one more really big reason to only ground an electrical system at one point. If your house system doesn't have a second negative to chassis ground connection, then your scenario would cause no problem.
 

Batvette

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And that is one more really big reason to only ground an electrical system at one point. If your house system doesn't have a second negative to chassis ground connection, then your scenario would cause no problem.
True enough, but whats our circuit reference ground? The vehicle chassis, not the battery negative post. The battery is the power supply of our circuit.
Also a second battery negative to chassis cable would prevent this scenario.
The grounding at one point I agree with. Thats anywhere on the vehicle frame if its a pickup (not body panels) and virtually anywhere on a unibody.
 

HaldorEE

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True enough, but whats our circuit reference ground? The vehicle chassis, not the battery negative post. The battery is the power supply of our circuit.
Also a second battery negative to chassis cable would prevent this scenario.
The grounding at one point I agree with. Thats anywhere on the vehicle frame if its a pickup (not body panels) and virtually anywhere on a unibody.
If you are using the chassis ground for return currents then you have no choice, connect your battery negative to chassis ground.

I am not even tempted to do this, my conversion is of a Ford Econoline and I am able to concentrate most of the DC stuff in a single location. Point of use DC loads are wired with duplex cable. I am attempting to follow ABYC standards for the DC equipment. Because I like it when stuff works.


AC wiring is all per NEC.

Would you use Chassis Ground for your 120V neutral (white wire)? It gets tied to chassis ground at a single point too.
 

Batvette

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If you are using the chassis ground for return currents then you have no choice, connect your battery negative to chassis ground.

I am not even tempted to do this, my conversion is of a Ford Econoline and I am able to concentrate most of the DC stuff in a single location. Point of use DC loads are wired with duplex cable. I am attempting to follow ABYC standards for the DC equipment. Because I like it when stuff works.


AC wiring is all per NEC.

Would you use Chassis Ground for your 120V neutral (white wire)? It gets tied to chassis ground at a single point too.
No, what little AC use I have goes through an inverter.
I used to have a workshop in a very old house with bad wiring and it was kind of funny I could measure 70 volts AC when I touched my finger to a meter and then to the ground. Had a big metal workbench, couple of times i touched that when I was wet and barefoot out of the shower.
Thatll wake you up.


Back to our discussion all of my accessory DC loads on the house battery circuit are using the same chassis ground. Using a negative return back to their power supply just makes the circuit longer and increases the chance for voltage drop. I do use two conductor cable when there's not a chassis ground in the vicinity of the load accessory.
 
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