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)
