Cell terminal bolt torque - you might be wrong!

fafrd

Photon Sorcerer
Joined
Aug 11, 2020
Messages
2,192
I do not quite follow what's happening with terminals, by the Eve description, it's clear (for me) that 8 N.m is the internal resistance to torsion of the terminal.
Just using between 3 and 4 N.m should be sufficient and with a comfortable distance of the "not to be used" 8 N.m., is there a need to go higher...?
3 N.m should be like 300gr on a one meter stick.
Good questions (to which I do not know the definitive answer),

I believe someone on the forum torqued to 80 inch pounds and damaged their terminal so going past 8Nm / 70 inch-pounds is clearly a very bad idea.
 

Gazoo

Dumb Dumb
Joined
May 12, 2020
Messages
2,794
Location
Texas
It was determined earlier 4.5 to 5.5 nm was enough. I can find where it was posted. Another one I should have bookmarked.
 

fafrd

Photon Sorcerer
Joined
Aug 11, 2020
Messages
2,192
It was determined earlier 4.5 to 5.5 nm was enough. I can find where it was posted. Another one I should have bookmarked.
Would be great if you can find the source. Do you recall the problem that materializes if you torque to less than 4.5 or 5.5Nm (40 to 48.7 inch-lbs)? High resistivity and/or variable resistivity?
 

cinergi

1.21 Jigawatts
Joined
Aug 9, 2020
Messages
1,157
I do not quite follow what's happening with terminals, by the Eve description, it's clear (for me) that 8 N.m is the internal resistance to torsion of the terminal.
Just using between 3 and 4 N.m should be sufficient and with a comfortable distance of the "not to be used" 8 N.m., is there a need to go higher...?
3 N.m should be like 300gr on a one meter stick.

8Nm is how much the terminal can be twisted, *not* the torque limit of a 6mm bolt.

Various tables show that M6 in aluminum can handle ~5Nm. Even SS bolts M6 are less than 8. So aluminum definitely can't handle 8.

4Nm torque != 4 newtons of force on the terminals. You have to run that through a calculator based on the bolt size. In this case, 4Nm works out to about 700 POUNDS of force against the terminal.
 

fafrd

Photon Sorcerer
Joined
Aug 11, 2020
Messages
2,192
The first post of this thread answers these recent questions ;)
I appreciate your reminder to review the excellent information you provided in the first post, but it’s not directly answering the question.

You’ve made clear why 4Nm / 35inch-lbs @ M6 is the upper limit of force/torque that should be applied to the aluminum terminals threaded with M6 threads.

What is not clear to me is why so many members seem to believe that that is an insufficient level of torque to deliver a good electrical connection?

What is driving this desire to achieve stratospheric levels of torque on these 280Ah LiFePO4 cell terminals?
 

cinergi

1.21 Jigawatts
Joined
Aug 9, 2020
Messages
1,157
I appreciate your reminder to review the excellent information you provided in the first post, but it’s not directly answering the question.

You’ve made clear why 4Nm / 35inch-lbs @ M6 is the upper limit of force/torque that should be applied to the aluminum terminals threaded with M6 threads.

What is not clear to me is why so many members seem to believe that that is an insufficient level of torque to deliver a good electrical connection?

What is driving this desire to achieve stratospheric levels of torque on these 280Ah LiFePO4 cell terminals?

Ah. My take:
  • Many people are still misinformed from the original 8Nm which we proliferated here for quite some time
  • 4Nm "feels" like nothing when using a socket wrench
  • People don't understand that 4Nm is 700 pounds of clamping force (700 pounds of weight on top of the terminal) in this case
  • People mistake their observed poor performance as insufficient torque when it's unclean or deformed terminals and bus bars
  • Paranoia, especially in a mobile situation
  • Edit: Maybe confusing the cell terminals of, e.g., CALB cells to these which have monstrous terminals by comparison
 
Last edited:

Luthj

Photon Sorcerer
Joined
Jul 1, 2020
Messages
945
I think they are referring to the clamping force when using a 6mm fastener.
 

cinergi

1.21 Jigawatts
Joined
Aug 9, 2020
Messages
1,157
4Nm is ~35 inch/lbs. Note the inch, not foot. Where are you getting 700 pounds?

No disagreement with your other points, of course.

Yes, clamping force (see original post). It's like there's 700 pounds of pressure pushing down on the bus bar to keep it in contact with the cell terminal.
 

fafrd

Photon Sorcerer
Joined
Aug 11, 2020
Messages
2,192
Ah. My take:
  • Many people are still misinformed from the original 8Nm which we proliferated here for quite some time
  • 4Nm "feels" like nothing when using a socket wrench
  • People don't understand that 4Nm is 700 pounds of clamping force (700 pounds of weight on top of the terminal) in this case
  • People mistake their observed poor performance as insufficient torque when it's unclean or deformed terminals and bus bars
  • Paranoia, especially in a mobile situation
  • Edit: Maybe confusing the cell terminals of, e.g., CALB cells to these which have monstrous terminals by comparison
Didn’t know about that history. Thanks for the explanation.

And I tend to agree that my one experience with 35inch-pounds of torque resulted in nice uniform connections that I was happy with.

Is these not some standard measure to determine whether your connection is ‘good enough’ in terms of mOhms?

My cells have IR of 0.25mOhms (or 2-3 times that based on what I measure using the dV/dA method; my solid bus bars have terminal-to-terminal resistance of 0.055 mOhms; and then there are two contact resistances to consider.

Pumping 80A through 0.25 + 0.055 + 2xRc should give me a voltage drop of 20mV + 4.4mV + 0.160Rc mV

Isn’t there some reasonable upper-limit how many mV you should measure above 25mV to decide that a connection is ‘good enough’?

For example, contact resistance exceeding the fixed resistances seems like a bad outcome (Rc > 0.1525 mOhms or total voltage drop @ 80A > 50mV)

while contact resistance < 10% of the fixed resistances seems pretty good (Rc < 0.03 or total voltage drop @ 80A < 27.5mV).

Are there any guidelines like that which can be tested at high current to decide if a connection is low-enough resistance?
 

Gazoo

Dumb Dumb
Joined
May 12, 2020
Messages
2,794
Location
Texas
Would be great if you can find the source. Do you recall the problem that materializes if you torque to less than 4.5 or 5.5Nm (40 to 48.7 inch-lbs)? High resistivity and/or variable resistivity?
This is the post. Took me long enough to find it...lol. Don't know if it helps much.
 

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
8,353
This is the post. Took me long enough to find it...lol. Don't know if it helps much.

Nice info.
That link is about clamping copper-clad aluminum busbars.
They show two bolts (which keeps the busbars from moving and loosening the screws.
The 4.5 Nm torque of 6mm is with lubricant (big difference in friction and clamping force vs. dry)

Contact only taking place at about 1% of the overlap area!

"6.3.2.1 Condition of Contact Surfaces

In practice, an electrical contact between the solids is formed only at discrete areas within the contact interface and these areas (known as ‘a-spots’) are the only current conducting paths. The a-spots typically occupy an area of the order of 1% of the overlap area.

Obviously, the larger the number of a-spots, the more uniform the current distribution across the joint area will be. This can be encouraged by ensuring that the surfaces of the conductors are flat and roughened (which removes the oxide layer and produces a large number of asperities) immediately before assembly. As the contact pressure is increased, the higher peaks make contact, disrupt any remaining surface oxide and form metal to metal contact.

In some areas an oxide film may remain. Copper oxide films on copper form relatively slowly and are semiconducting because copper ions diffuse into the oxide layer. When copper oxide films are compressed between two copper surfaces, diffusion can take place in both directions so conduction takes place in both directions. This is very different from aluminium, where the oxide is a very good insulator and forms within microseconds of exposure to air.

Since the area of each a-spot contact is small, the current density is high, leading to higher voltage drop and local heating. In a well-made joint this heat is quickly dissipated into the mass of the conductor and the temperature of the interface will be only slightly above that of the bulk material. However, if the contact pressure is too low and the joint has deteriorated, local over-heating may be enough to induce basic metallurgical changes including softening and melting of the material at the a-spot. At first sight this may appear to be advantageous, however, as the joint cools the material contracts and fractures and is subsequently liable to oxidise.

Since elevated temperature is the first symptom of joint failure, maintenance procedures should be established to monitor the temperature of joints with respect to that of nearby bar using thermal imaging. If, under similar load conditions, the differential temperature increases, it may be a sign of early joint degradation. As a first step, more intensive monitoring should be undertaken and, if the trend continues, remedial action taken.

It is not normally recommended that the surfaces of copper-to-copper joints are plated unless required by environmental considerations. In fact, plating may reduce the stability of the joint because, as soft materials, the plating may flow at elevated temperatures leading to reduced contact pressure.

However, to ensure a long service life, a contact aid compound is recommended to fill the voids in the contact area and prevent oxidation or corrosion. Many proprietary compounds are available or, if none are available, petroleum jelly or, for higher temperatures, silicone vacuum grease may be used.

6.3.2.2 Effect of Pressure on Contact Resistance

Joint resistance normally decreases with an increase in the size and number of bolts used. Bolt sizes usually vary from M6 to M20 with either four or six bolts being used. The appropriate torque for each bolt size depends on the bolt material and the maximum operating temperature expected.

Contact resistance falls rapidly with increasing pressure, as shown in Figure 69, but above a pressure of about 30 N/mm² there is little further improvement. In most cases it is not advisable to use contact pressures of less than 7 N/mm², with pressures above 10 N/mm² being preferred. The contact resistance for a joint of a particular overlap area is obtained from Figure 69 by dividing the contact resistance for 1 mm2 by the overlap area in mm2 ."

1611897525558.png
 

fafrd

Photon Sorcerer
Joined
Aug 11, 2020
Messages
2,192
This is the post. Took me long enough to find it...lol. Don't know if it helps much.
I just linked to an article on Aluminum lugs in one thread or another that stayed over tightening bolts beyond recommended torque levels results in stretching the bolt and degrading the connection.

So I’m guessing torquing an M6 bolt above 5.5Nm means it’s been damaged...

Now, then there is the angle of which metal the bolt is composed of, so there are a lot of details to suss out here...
 

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
8,353
Clamping force vs. torque is an engineering endeavor in and of itself.
I've seen figures for dry vs. lubricated vary 4:1
This article says, "Don't trust tables, measure it for yourself"

 

Honuz

Solar Enthusiast
Joined
Oct 22, 2020
Messages
158
8Nm is how much the terminal can be twisted, *not* the torque limit of a 6mm bolt.

Various tables show that M6 in aluminum can handle ~5Nm. Even SS bolts M6 are less than 8. So aluminum definitely can't handle 8.

4Nm torque != 4 newtons of force on the terminals. You have to run that through a calculator based on the bolt size. In this case, 4Nm works out to about 700 POUNDS of force against the terminal.
Yes, that's what I said about the terminal resistance to torque, 8 Nm.
There are different qualities in bolt, a stainless steel 8.8 M6 will take...11Nm (staying in the 80% elasticity limit) of torque, it's written on the head of the bolt. Also depend on the threading of the bolt. (and yes lubrification as a great influence)
But anyway, the terminal would be the weak part because of the aluminum.

4 Nm is not different then 4 N of force at 1m of the axle.
Torque is distance x force : 4 Nm = 4 N at 1 m = 8 N at 0.5 m = 800 N at 0.005 m .. The force needed to be applied depend on the distance. No need calculus with a torque wrench (dunno if it's the correct word, sorry not my mother tongue).

at 0.005 m, 5 mm of the axle the 800 N would translate in 170 Pounds of force (perpendicularly to the bolt), but this measure is kind...useless cause this force will tend to infinity at the axle center.
Reason we got Nm to be able to compare.
The only way to put those bolt is to use a torque wrench or be gentle on the force applied. I would more say....put the bolt in contact then do 45 to 60° angle rotation...? An M6x1 will penetrate 1mm per 360° an M6x1.25, 1.25mm.
 
Last edited:

cinergi

1.21 Jigawatts
Joined
Aug 9, 2020
Messages
1,157
at 0.005 m, 5 mm of the axle the 800 N would translate in 170 Pounds of force (perpendicularly to the bolt), but this measure is kind...useless cause this force will tend to infinity at the axle center.
Reason we got Nm to be able to compare.
The only way to put those bolt is to use a torque wrench or be gentle on the force applied. I would more say....put the bolt in contact then do 45 to 60° angle rotation...? An M6x1 will penetrate 1mm per 360° an M6x1.25, 1.25mm.

I'm not sure if you're talking about my 700 pound figure or if you're just discussing general torque numbers ... but to be clear, my 700 pound figure has no length qualifier in the units - it's not torque. It's 700 pounds of downward force on the bus bar as a result of the bolt being torqued to 4 Nm. That's more than enough to make a good electrical contact if the surfaces are properly prepared.
 

Honuz

Solar Enthusiast
Joined
Oct 22, 2020
Messages
158
I'm not sure if you're talking about my 700 pound figure or if you're just discussing general torque numbers ... but to be clear, my 700 pound figure has no length qualifier in the units - it's not torque. It's 700 pounds of downward force on the bus bar as a result of the bolt being torqued to 4 Nm. That's more than enough to make a good electrical contact if the surfaces are properly prepared.
Nope, I never talked about your 700 pounds..😃
 

HLB

New Member
Joined
Sep 23, 2020
Messages
2
Good discussion. If you want to get the most clamping force you can for your terminal, thread, and bolt material; then you might just find a supposed similar aluminum or sacrificial battery, tap it such that your bolts fit with the same looseness and depth, lubricate with your preferred lubricant, gradually increase torque until it strips, do that several times, and if the results are within a reasonable range, then cut your torque to maybe half or a third for actual install. If you have a bad battery with a worn out hole, there should still be room to drill and tap a test hole next to it. Doing that similarly for withdrawal force would be a bit more complicated but doable. What about that?
HLB
 
Top