# Bus bar size and material



## evmetro (Apr 9, 2012)

I can't get the search function to work on my droid, so sorry if this is already discussed here. I got a pack of 45 100 ah cells, and the bus bars that came with them look too cheesy to use. (Dual 8 gauge crimped into wire lugs) I Want to make up some real bus bars, but am not sure what the best material is what size stock I should get. I have not seen copper stock at my local metal supplier, but I have plenty of steel laying around my shop... thoughts?


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## palmer_md (Jul 22, 2011)

I'd stick with copper.

http://www.onlinemetals.com/merchant.cfm?id=253&step=2&top_cat=87


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## dougingraham (Jul 26, 2011)

Use copper. Look for copper bar stock on Ebay or buy from Mcmaster Carr. For intercell connections it is considered a good idea to have some flex in the connection so copper braid is a good idea. There is at least one thread here that discusses a DIY solution. You can get a good off the shelf solution from the EVTV store.

If you choose to make your own from bar stock it is a good idea to tin plate the material after you cut and drill it to length. I would suggest several thin plates with a bend in the middle to make it somewhat flexible. You can insulate this with heat shrink tubing.

Best Wishes!


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## major (Apr 4, 2008)

dougingraham said:


> Use copper. Look for copper bar stock on Ebay or buy from Mcmaster Carr. For intercell connections it is considered a good idea to have some flex in the connection so copper braid is a good idea. There is at least one thread here that discusses a DIY solution. You can get a good off the shelf solution from the EVTV store.
> 
> If you choose to make your own from bar stock it is a good idea to tin plate the material after you cut and drill it to length. I would suggest several thin plates with a bend in the middle to make it somewhat flexible. You can insulate this with heat shrink tubing.


Good. Stay away from ferrous metals. Copper is best. Have seen aluminum used but it likes to oxidize. Here's some I made in plated copper. The unplated one is from TS. 










I made a die to put the hump in the middle. And it works nicely to use laminated bus of maybe 3 or 4 layers. You'll find copper much easier to punch than drill. Because it is soft, sheet metal tools can work even up to 1/8 inch thick. Plating isn't necessary, but nice in the long run. Tin is easy for in house, or send them out for nickel plate.


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## Joey (Oct 12, 2007)

You can get the resistivity of different materials at Wikipedia: http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity

If you divide the resistivity by the cross section area of the bus bar you can get resistance per meter. 

I did an example for a few metals. If the bus bar is 25 mm wide, and your circuit is 10 meters long, I figured out how thick the bus bar would have to be for 2% line loss in the conductor, given 200 amps average current and a 200 V system. (This is just one way to make a comparison.)







I would stick with copper because there is no good way to increase the contact area between your battery terminal and bus bar.


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## evmetro (Apr 9, 2012)

I ended up finding some copper sheet metal at another metal store, and had them chop off a bunch of 1 inch strips to basically make 1 inch flat stock. I will just need to cut them down to length and put the holes in. I figured I will try these in a few layers. They seem pretty flexible. Major, that die looks pretty cool. I may have to try my hand at making one of those. Thanks all for all of your replies and info. I think I can see now why people steal copper.. My little handful cost a hundred bucks!


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## major (Apr 4, 2008)

evmetro said:


> I ended up finding some copper sheet metal at another metal store, and had them chop off a bunch of 1 inch strips to basically make 1 inch flat stock. I will just need to cut them down to length and put the holes in. I figured I will try these in a few layers. They seem pretty flexible. Major, that die looks pretty cool. I may have to try my hand at making one of those. Thanks all for all of your replies and info. I think I can see now why people steal copper.. My little handful cost a hundred bucks!


Not hard to make and works well in a vice or arbor press.


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## Sunking (Aug 10, 2009)

major said:


> Copper is best.


Nah Silver is better.


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## PStechPaul (May 1, 2012)

Silver has only 6% better conductivity than copper, but costs many times as much. It is also 17% higher density, so copper wins when weight is a major factor. "Best" is always a relative term, just as lithium batteries may or may not be "better" than lead.

Silver plating of copper may be better than nickel, and certainly better than leaving it bare. Copper oxide is an insulator, while silver sulphide (oxide is not the common black tarnish) is much more conductive, so oxidation does not cause as much conductivity loss. A thin silver plating can be applied by hand even without electrical current, and does not cost much more than nickel. And being softer, silver and copper will deform under pressure and create more conductive surface area.

You can probably make inexpensive bus bars with copper pipe, especially if you can get some scrap from a plumber or recycler. You need only flatten the ends where they bolt to the terminals. Type L copper is heavier than type M, and type K is even heavier (about 1/8" walls for 3/4" and 1" pipe):
http://sizes.com/materls/pipeCopper.htm


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## evmetro (Apr 9, 2012)

I tried my hand at making the bus bar jig...




























I have been putting three layers, each is about 1 MM thick and about 25 MM wide, but I am not sure if this is enough. I may have enough for a fourth layer, but I will have to see what I have left after I get three all around. The spec sheet for the cells says 3c/5c, so I need these to be good for 300 amps continuous.


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## palmer_md (Jul 22, 2011)

Nice work. It looks really good. Once you figure out how many to stack, just shrink wrap them to keep them together and also help to avoid unnecessary exposed conductors.


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## PStechPaul (May 1, 2012)

A 3/16" x 3/4" bus bar is rated at 270 amps for 30C rise in free air. It's 0.14 square inch cross section. Your 3mm x 25mm is 0.12 square inches, so probably there will be a higher temperature rise. It's about equivalent to 2/0 wire, which is rated about 390 amps based on cross sectional area where #10 AWG is good for 30 amps. Each 3 wire gauges doubles the capacity, so #7 is 60 amps, #4 is 120 amps, #1 is 240 amps, and #3/0 would be 480 amps. But in free air the temperature rise is based on surface area/watt, and using that metric I get 305 amps for the 3/16 x 3/4 bus bar. You can get even higher current rating by separating the conductors and allowing (or forcing) air flow.

Another way to think about it is watts/foot at rated current and temperature. The 3/16 x 3/4 bus bar is 16 watts/foot at 25C and 23 watts/foot at 100C, for 523 amps. You can also look at the bus bar as a heat sink based on surface area and a thermal resistance to free air, which might be about 5C/watt for your links. Thus a 6" piece might dissipate about 5 watts at 300 amps with a temperature rise of 25C.

Best thing might be to run current through it and measure the temperature. You can make a 1000 amp current source using a single turn in a 500W toroid or a microwave oven transformer.

I have a wire and bus bar size calculator on my website:
http://pstech-inc.com/WireSize.xls


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## evmetro (Apr 9, 2012)

The bus bars will be inside the car and very easy to access, so I can shoot them with my temp on a test drive. If they heat up, I will add another layer.


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## Yabert (Feb 7, 2010)

evmetro said:


> I have been putting three layers, each is about 1 MM thick and about 25 MM wide, but I am not sure if this is enough. I may have enough for a fourth layer, but I will have to see what I have left after I get three all around. The spec sheet for the cells says 3c/5c, so I need these to be good for 300 amps continuous.


Nice bus bar!
You will be fine because you won't need 300A continuous.... simply because you won't need over 40 Kw to move a small car. The battery current will probably be around 50-150A most of time (excluding accelerations). 
Except maybe if you maintain 100 mph on highway!!


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