copper vs aluminum buss

[Farfle]

Hello all, I was looking for a material to do my bikes inter-battery buss bars with, and its come down to either aluminum or copper. The plan is to use well crimped ring terminals on the turnigy packs, and go directly to a buss bar with threaded holes.

Now I know copper has the edge as far as conductivity per volume, BUT as far as conductivity per weight, aluminum wins IIRC. so, I guess my questions are these:

which of the two is the most machineable (need to drill and tap 50+ holes)
which holds threads the best?
what are the corrosion properties of both?
Is the copper worth it at 8 bucks a foot compared to 2 bucks a foot?

Thanks in advance

 

[Chalo]

If you use aluminum, you'll need to use an electrical joint compound (like Alcoa #2). Those are kind of nasty and inconvenient to apply, since you have to abrasively scrub each connection at the time of application. Aluminum's conductivity is good, but the resistance of its passivated skin layer is a problem.

Common aluminum alloys are easier to machine than electrical copper, but their conductivity usually falls along with increasing machinability. Commercially pure aluminum is just about as gummy as plain copper. But you could use a common structural or architectural alloy like 6061 or 6063, and just use plenty of it.

 

[nicobie]

Machinists hate copper. Messes up tooling bad. But if all you have to do is drill and tap, it might be ok. I'd test on a small piece first.

 

[Chalo]

Machinists hate copper. Messes up tooling bad. But if all you have to do is drill and tap, it might be ok. I'd test on a small piece first.

I always liked copper just fine when I worked as a machinist. Regular fluted taps are no good for copper, though. Spiral point taps are better, and forming taps are best.

 

[liveforphysics]

The ultimate solution for a bus where you aren't volume constrained would be aluminum. This is because it's cheaper and lighter than copper.

However! It takes lots of fiddly testing and refining surface prep processes before mating a copper tab to, and aluminum bus connection compounds to try to delay dissimilar metal corrosion and passivization layer oxide formations that impede your conductivity etc.

You just just want to hit a home run without creating a connection environmental long-term degradation/oxidation program to perfect each material and surface prep and anti-oxidizing coatings etc, just use copper. If you use copper, you just need a good copper terminal (tinned with some long-term copper corrosion resistant plating), a smear of a grease like de-oxit or various other dielectric greases for copper/copper connections, and then mate it together with good clamp load and you just hit your easy home-run at the cost of a little bit more weight and cost.

For doing 1-off stuff that you want to last and be durable, I would work in copper. For doing something you want to move to production and you have the time and budget to do HALT testing for future corrosion issues down road in that joint, you can do aluminum and end up with great results after you've figured out all the processes.

 

[liveforphysics]

Machinists hate copper. Messes up tooling bad. But if all you have to do is drill and tap, it might be ok. I'd test on a small piece first.

Modern high-speed machining of copper tends to gum up and fail and be a nasty mess.

Slow old-school machining processes on copper tend to cut really nicely and leave a beautiful clean finish.

 

[Chalo]

For doing 1-off stuff that you want to last and be durable, I would work in copper. For doing something you want to move to production and you have the time and budget to do HALT testing for future corrosion issues down road in that joint, you can do aluminum and end up with great results after you've figured out all the processes.

The other situation where I would lean towards aluminum is where the busbar becomes the primary structural element of the pack.

 

[Sancho's Horse]

I am using aluminum attached mechanically to A123 packs. This is advisable in this sort of situation, because the bars between the tabs are all aluminum. The real problems obviously come with mating dissimilar metals. Not just because of corrosion, because a little scraping and a little quick greasing can solve that, but anywhere there is a mechanical fastener mating aluminum and most any other metal, there are some significantly different rates of thermal expansion which typically occur. This is where the potential bad stuff starts to happen. Aside from mechanical breakdown of fastening surfaces, i.e. stripping threads etc., there can be deterioration of mated surface contact that might not be easily perceived (especially where there is extensive solder, or the like, filler metal mating), The separation lead to oxidation and will lead to build-up of heat in that area. This can first present itself with catostrophic failure. That is the worry most have about using aluminum (aside from the pain that near instantaneous oxidation presents). You eventually have to marry it to copper, and if that can be done so that it is reliable and not perturbed, you are well on your way. So if your pack already uses aluminum, an aluminum bus bar just pushes the copper mating further down the line, but if everything else is copper don't use aluminum to save weight or cost, way to complicated and too much hassle. Unless you are going production, as Luke says.

As far as machining goes, I wish I had that problem. However, I have machined aluminum extensively with a simple router, and even table/miter saws and it works very reliably. I like the looks of the Ogee edge . Tapping requires considerably more removal of shavings. The taps go in and cut easily, but it is deceptive, because it will make you want to push the envelope, and has resulted in broken taps to my initial surprise.

 

[Farfle]

Thanks guys, copper it is then so heres the tentative procedure:

use an appropriately size hole tin plate panduit ring terminal.

scuff the buss and terminal with a SS wire brush

apply dielectric grease to buss

Apply permanent strength loctite 262 to bolt

Bolt through the ring terminal using a stainless cap headm6x1x10 hex head with a good washer and torque appropriately.

Anybody have torque tables for short metric bolts into copper? :lol: :lol:

 

[jonescg]

When the thread hits home, no more than a quarter turn or you'll strip it. I've threaded 2 mm copper buss before - it worked, but it wasn't pretty. Then again, not much that comes out of my shed looks pretty :lol: Just use stainless steel screws, not zinc plated ones!

Aluminium as a bussing material is only worth it if you are building a substation or something where space isn't an issue, and even then the corrosion blows.

 

[Njay]

Are there taping tools specifically for copper? I mean, the usual tools found on stores will do the job on copper? I have the 3 tools of increasing thread deepness.

 

[jonescg]

I wouldn't tap something you couldn't pop out the other side. The shavings don't break off like they do with harder metals. But cutting tools for soft metals tend to have steeper angles so the swaff clears more easily, and so the tool doesn't pull through.

 

[999zip999]

Can always solder to copper. Both grip a tap and easy to snap.

 

[Chalo]

Ultra-ductile metals like copper benefit from forming taps. You drill closer to net size, and then cold forge the thread in there.

 

[flathill]

No matter the surface prep I would never make a Cu to Al connection
Unless It was fused
someone should
low cost ultrasonic or laser welder on kicksarter

The exception would be if it was cca wire/tab/bar
Copper clad Al

 

[Farfle]

I have used a forming tap before, ill pick one up when I order the copper. What do you guys think the thinnest I can go on the buss is? The problem is the buss's height, as the battery is 12.5" long + wires, and the battery cage is 13.5" long. so both wiring ends have to be .5" or less. Who was the bonehead that forgot the wire ends when he modeled the battery into the battery cage

Here is a picture of the battery. The ends are where the buss bars will be.

 

[TMaster]

You may need some tap magic cutting fluid for putting on the tap when you tap the holes. I have never tapped copper, but I have tapped steel and aluminum and both needed it. Theres a few versions, for aluminum and for all metals. Both would work.

I have machined copper, and it didn't seem as hard to mill as 6061 aluminum. 2 flute solid carbide endmill.

 

[boostjuice]

Just to clear things up, the metallurgical property that makes copper a nightmare to machine is its abrasiveness (theres a reason why they blend it into many types of brake pads). It's deceptive because its so soft, but It bluntens cutters quickly and jams under pressure because of its malleability (ie tapping holes). To give an example of just how abrasive copper is, I pretty much buggered a brand new 6mm HSS drill from 32 holes through 1mm copper sheet.