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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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Hi. I assume you need more than one of these? Rather than try to cut one out of a sheet, do you have the option of paying someone with a brake to bend one for you out of 3mmx25mm?

Jim
 

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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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