Excellent Lee, thanks. I am not arguing, just speculating - I bet if you tighten the first time until the resistance drops no more, then dis and re assemble the same joint again, tighten until the resistance stops dropping- you will get a different final resistance and a different final torque. And the resistant might very well go down the second time - without any sanding.
Me - having been surprised many times after doing statistical work ups on procedures like this - I think you have to be more rigorous than a one time trial. Maybe that has been done? However, I was leaving room in my comments that maybe the thickness of the terminals oxide layer could be reduced and that might be a change in the right direction. I would hesitate to use any sand paper harsher than say 320, or 400 grit. We are talking about cleaning it, not removing much and certainly not changing the shape. V = IR = 0.001 Ohms x 100 A = 0.1 volts of drop. I^2R = 100^2 x 0.001= 10 Watts. Am I wrong? Still though even 10W is a lot times however many terminals in the pack...is still hot On Thu, Jul 31, 2014 at 2:48 PM, Lee Hart via EV <[email protected]> wrote: > Michael Ross wrote: > >> In air, aluminum oxide forms nearly instantly. Therefore, sanding >> is a useless activity, if the goal is to remove aluminum oxide... >> > > You're right; aluminum oxide forms very quickly. However, the longer it is > exposed to air, the thicker the insulating layer gets. So cleaning the > terminal to remove the oxide immediately before assembly minimizes the > thickness, and thus *does* reduce the resistance. > > Very thin insulating layers behave strangely. First, the very thin oxide > layer is porous; it has lots of holes. Like spray painting something; > before you have enough paint to completely cover, you can still see the > underlying surface through the holes. With enough contact pressure, the > metal can deform in to fill these holes to make contact anyway. > > Electrons can also "tunnel" across very small gaps even when there is an > insulator in the way. The contact resistance doesn't go from 0 to infinite > as soon as there is a tiny layer of some insulator; it gradually rises as > the layer gets thicker. > > I don't like the idea of sanding terminals. You want then to have the >> flat machined surface they have leaving the factory o get a good bolted >> joint with as much contact area as possible... >> > > What you think is a flat machined surface is actually a mountain range > under a microscope. Machining, sanding, polishing etc. just reduces the > scale of the mountains. > > When the two surfaces touch, only the peaks actually make contact. > Increasing the contact pressure makes the metal deform, flattening the > peaks, and improving the contact area. The deformations also break any > oxide layer that may have formed, if it's thin enough and weak enough. > (Aluminum oxide is a tough one, because it grows strong and thick). > > If you're bolting together steel, the contact pressures needed to deform > it are tremendous. But lead, copper, silver, gold, and aluminum are all > very soft metals -- it takes a lot less contact pressure to make them > deform to improve the contact. > > > I suppose one might prove whether the resistance is changed for the >> better if you have a really good instrument to check it. But this >> will not be your garden variety multi-meter. >> > > It's pretty easy to measure what's happening yourself. The test is not > difficult. I would urge people to try it themselves. It's especially > enlightening with hard-to-connect metals like aluminum. > > You need a digital multimeter with a millivolt scale (usually 200mv or > 400mv full-scale). And, you need a source of a known DC current of an amp > or more. A 10-amp battery charger with a ammeter will do. > > Let's say you want to measure the resistance of the connections to a 12v > battery: Run the battery down, so it will actually charge at 10 amps. > Connect the charger at a point somewhat away from the battery, so the will > be current is flowing in the wires and terminals you want to check. Set > your meter to its millivolt scale. Connect one lead to the post of the > battery itself. Connect the other lead to the terminal that connects to > this post. > > Read the millivolt drop of the terminal, and the charging current from the > charger. Use Ohm's law to calculate the resistance. For example: > > R = V / I = 10 millivolts / 10 amps = 1 milliohm (0.001 ohms) > > Under normal circumstances, 0.001 ohm would be a good connection. But it's > a *bad* connection in an EV traction pack! At 100 amps, it would have a 1 > volt drop, and so produce 100 watts of heat! > > Chinese lithiums I've tested straight from the factory are this bad, and > sometimes worse! > > If you don't believe that cleaning, bolting, and contact "greases" help, > try an experiment. > > 1. Get two pieces of aluminum that's been sitting around a long time. > Bolt them together. Measure the torque if you can; if not, use a > socket wrench and apply a "know" force. > > Measure the resistance between them (as described above). Notice > that the tighter the bolt, the lower the resistance (to a point; > then it doesn't matter any more). > > 2. Take them apart. Clean the two surfaces with sandpaper, file, > wire wheel, etc. Clean off any resulting dust. > > Bolt them together again, and measure the resistance again at > several different bolt torques. You will find that the resistance > is lower, at every bolt torque (though it still reaches a point > where more torque doesn't reduce resistance). > > 3. Add any kind of contact "grease". Noalox, axle grease, vaseline, > etc. Repeat the test. You will find no difference in resistance, > with or without the grease, no matter which one you use. > > But... leave the bolted pieces of aluminum outdoors for a while, > where they will get hot/cold/wet/dirty etc. Without the grease, > the contact resistance will go up. With the grease, it will stay > about the same. > > This is a complex subject. I hope I have not oversimplified it too much. > The experts already know it, and can ignore my analogies. But I hope those > with only a little knowledge may gain some understanding. And, I hope > people will *measure it for themselves*. That's far better than listening > to experts debating how many electrons can dance on the head of a pin. :-) > > -- > The definition of research: Shoot the arrow first, and paint the target > around where it lands. -- David Van Baak > -- > Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/ > group/NEDRA) > > -- Put this question to yourself: should I use everyone else to attain happiness, or should I help others gain happiness? *Dalai Lama * Tell me what it is you plan to do With your one wild and precious life? Mary Oliver, "The summer day." To invent, you need a good imagination and a pile of junk. Thomas A. Edison <http://www.brainyquote.com/quotes/quotes/t/thomasaed125362.html> A public-opinion poll is no substitute for thought. *Warren Buffet* Michael E. Ross (919) 550-2430 Land (919) 576-0824 <https://www.google.com/voice/b/0?pli=1#phones> Google Phone (919) 631-1451 Cell (919) 513-0418 Desk [email protected] <[email protected]> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20140731/c1a29cad/attachment.htm> _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
