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Discussion Starter · #1 ·
Hi Folks,
I'm making some bus bars to join together 400v tesla pack in a diffirent physical layout to original so i need new bus bars.
I happened across a sheet of 4mm stainless steel and i was pretty excited to use it to make bus bars. mostly because it wont corrode easily.

A quick look on the internet reveals stainless steel has a higher electrical resistance.
Cooper for example has a resistivity of 1.68×10−8
Aluminium 2.65×10−8
carbon steel 1.43×10−7 (note the minus 7 not minus 8)
stainless steel 6.90×10−7 (note the minus 7 not minus 8)

So i can easily imagine copper is the most desirable choice. That said is Stainless steel still a problem? should i just make them slightly larger to compensate?
Original was 25mmx3mm (75mm2) how much larger would be desirable? 25mmx4mm (100mm2)?

Aluminium is easy enough for me to get but i thought since i have the stainless, why not?
 

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A quick look on the internet reveals stainless steel has a higher electrical resistance.
Cooper for example has a resistivity of 1.68×10−8
Aluminium 2.65×10−8
carbon steel 1.43×10−7 (note the minus 7 not minus 8)
stainless steel 6.90×10−7 (note the minus 7 not minus 8)

So i can easily imagine copper is the most desirable choice. That said is Stainless steel still a problem? should i just make them slightly larger to compensate?
Sure. Just make them 69/1.68 = 41 times as high cross section!

Original was 25mmx3mm (75mm2) how much larger would be desirable? 25mmx4mm (100mm2)?
41 x 75 = 3080 mm², say 100 mm x 31 mm. Or to retain the original width, a thickness of 3080 / 25 = 123 mm. You'd need really, really long bolts, and drilling 123 mm stainless steel would be a bitch.

Now you can see why people don't make battery straps out of stainless steel.

I've never even heard of aluminium straps; always copper. Aluminium is brittle.
 

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Discussion Starter · #3 ·
well that settles it. great info thanks!
Just confirming, would it be 6.9/1.68 = 4.1 instead of 69/1.68=41?
in either case, thats a huge stainless steel strap.

I priced some copper yesterday. I need 4mm x 70mm x 300mm to cut the shape i need for each strap. the closest i could get was 5mm x 90mm in 4 metre length for $282!
I'll see if i can change the design to make them smaller.
 

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A quick look on the internet reveals stainless steel has a higher electrical resistance.
Cooper for example has a resistivity of 1.68×10−8
Aluminium 2.65×10−8
carbon steel 1.43×10−7 (note the minus 7 not minus 8)
stainless steel 6.90×10−7 (note the minus 7 not minus 8)
Sure. Just make them 69/1.68 = 41 times as high cross section!
Just confirming, would it be 6.9/1.68 = 4.1 instead of 69/1.68=41?
No, as you pointed out when you first shared these value, the value for stainless steel is more than an order of magnitude greater.

thickness ratio (compared to copper):
r = (6.90×10^−7) / (1.68×10^−8)
r = (69.0×10^−8) / (1.68×10^−8)
r = 69.0/1.68
r = 41
(as Coulomb already said)
 

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If you make a stainless steel bus bar 4mm x 25mm and 100mm long, it will have a resistance of about 690 uOhms. If you are pulling 1000 amps that would be a voltage drop of 690 mV and power dissipation of 690 watts. The equivalent copper would be 16.8 uOhms and 16.8 watts.

You might be able to use copper water pipe for bus bars. Just use a press to flatten it for the terminations. 1/2" type L has a cross sectional area of about 0.08 in^2 which would make a 100mm bus bar with 33 uOhms resistance and a voltage drop of 33 mV and power of 33 watts at 1000 amps. 33 mV represents approximately 1% of the voltage of an LiFePO4 cell. 1/2" x 10ft type L pipe is about $14 at Home Depot, so you can make 30 bus bars for about 50 cents each.

It is a good idea to use silver plating (Kool-Amp powder) on bus bars for best conductivity of the joint. And some grease may also help avoid corrosion.
 

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Discussion Starter · #6 ·
Thanks Brian_, now i understand why the decimal point moved.

Thanks PSTechPaul, i was thinking of similarly dodgey ideas. Like collecting scrap copper and using my forget to make some forged copper bus bars. Cheaper, but way dodgy. I think given the amount im spending on the car, i dont think i should complain about $282 :)

THanks for your advice.
 

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I priced some copper yesterday. I need 4mm x 70mm x 300mm to cut the shape i need for each strap. the closest i could get was 5mm x 90mm in 4 metre length for $282!
I'll see if i can change the design to make them smaller.
And how about having them lasercut?
In The Netherlands there is a company that has up to 3 mm Copper (CU-DHP-R240) which they can cut. The challenge however is that single piece production is still quite expensive. Two of the same product is only a little more expensive than a single one. So it only helps if you can design in such way that you have multiple of the exact same busbars.
 

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Yes copper is more ductile but I can't see any good reason not to use Aluminium , Aluminium is used extensively as power transmission cables and bus bars.

I would definetly use it before stainless steel.

http://electrical-engineering-portal.com/copper-or-aluminium
One thing perhaps to keep in mind is galvanic corrosion
Further read on http://www.anzor.com.au/blog/galvanic-corrosion-keep-those-metals-apart/
My contactors have stainless studs en bolts and therefore I prefer to use copper over aluminium.
 

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well that settles it. great info thanks!
Just confirming, would it be 6.9/1.68 = 4.1 instead of 69/1.68=41?
in either case, thats a huge stainless steel strap.

I priced some copper yesterday. I need 4mm x 70mm x 300mm to cut the shape i need for each strap. the closest i could get was 5mm x 90mm in 4 metre length for $282!
I'll see if i can change the design to make them smaller.
3mm x 25mm should be just fine!!!!! What the heck are you thinking??

I found a 6mm bend allowance works.
 

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Discussion Starter · #12 ·
I knew stainless steel having a higher resistence would need to be thicker, which is why i initially suggested 4 x 25, but i didnt know to what scale. now i know its 41 times thicker! so not practical to use stainless steel.

The shape is a roughly a 'Z' on its side to create an angled bus bar to join stacked modules in series. I cant make that out of one piece without having a really wide piece of copper to start with. I dont want to solder or join the pieces, just to avoid complexity. Its a lot of wastage but i'll do it right the first time.
 

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this tin-plating - I had a quick look on utube and it looks like one has to electroplate the tin onto the copper? How difficult is this anyway? I'm not feeling too bad about using my anti-corrosive paste, but would like to know what is involved before I go ahead and do 'final' assembly.

I've done electroplating before, and to get a good result, one needs to heat the solution, have a bubbler, and it takes some time.

Hmmmm, maybe silver-plating onto copper is easier - should spontaneously react with copper.

Thanks.
 

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Late to the party but what did you settle on? Im about to do the same...I wasn't able to find what Teslas original bus bars are; they're heavy and appear to be nickel coated but can't use them anyway...Thinking with just going with copper wire and crimping the ends with those heavy duty kits. Dropping a 2016 Tesla in a sand rail 🤞
 

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One of my recent battery builds required some unusual terminals, which I figured I'd make, but I didn't have any copper stock of suitable size... I already had some experience melting things, so it turned out to be a piece of cake... some pictures below. Besides the obvious caveat with machining equipment being expensive, the actual copper element is pretty affordable - electric furnace can be purchased on Ebay for less than $200. I went the lazy route of making a very rough piece of stock with the intention to machine it, but it's possible to cast much nicer bus bars that are ready to use after minimum clean up. Source material for melting was scrap wiring, so definitely decent grade of copper in terms of conductivity.

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What did you use for molds?
Just two pieces of angle iron with two spacers in between. Spacers can be any flat stuff like square tubing or flat bar. Between thickness of spacers and the distance between them you can adjust the size of the resulting ingot. But like I said, this method is good enough when the intention is to machine some part out of the resulting metal. For something like bus bars, especially if need many at once, I'd use a more sophisticated mold. For my bus bars I opted just to buy flat stock, cut and drill it - got to pick the battles properly :)
 

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Making bus bars out of flat stock on a milling machine is probably the easiest thing. Rough cut the pieces, then trim them to length with an end mill. Then drill multiple at once, without even having to measure anything ahead of time.
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