# [EVDL] BusBar sizing



## EVDL List (Jul 27, 2007)

I am designing a battery module with 26650's tab welded to copper busbars.

I am trying to figure out the optimum size for these bars.

Charts show 500A at 65C rise would be 3/16 by 3/4. (I like that size)
Now I figure that is a continuous rating.

So what would the experts recommend for say 1000A 30 second rating?


Also Does anyone know a good source for busbars or 3/16 sheet to cut
them from? There are a lot of google sources but just wondering if
anyone here has experience/reccomendations with some companies in
particular.

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## EVDL List (Jul 27, 2007)

> Jeff Shanab wrote:
> > I am designing a battery module with 26650's tab welded to copper busbars.
> >
> 
> ...


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## EVDL List (Jul 27, 2007)

There will not be more than 42A per cell at 1000A of discharge.(26650's
the larger cells limit me to 500Amps and are 100Amps per cell max)
A little resistance may actually help balance current flow as it will
dominate the resistance of the cell. (gotta test that)
Someone else on this list is trying the paralleling of serial strings
with management, I will let him speak up if he wants.

Welds are smaller but have an extremely low resistance compared to
soldering. Lets please not go thru the solder vs crimp argument. We are
not gonna change each others minds. (Please see the archives if you are
interested) You will notice that soldering wires to a circuit board are
frowned upon by anyone with a service department for non disposable
electric hardware. The solder creeps up the wire and creates a break point.
The soldering requires getting the cell top and the tab to the same
temperature. Way to much gets soaked into the heatsink of the cell.
Welding offers the opportunity to marry the metal in a shorter time with
the heat localized at the weld site.
Furthermore, an inverter welder offers even more control over the amount
of energy that gets into cell. A much shorter pulse at a higher amp and
only the metal of the top and the tab are the heatsink. The serial weld
has two current paths. one thru the top of the cell and one thru the
tab, they come up to melt close to the same time.

If the cell puts out a lot of current momentarily a solder joint can fail.

AND it takes at least 10 times longer to solder.

I will weld 26650's or use the bigger cells with threaded ends. I just
want to be ready for whichever one comes in at a good price.
I am gonna use a micro and am gonna get started on the software side of
things.
I have 23 years experience making molds for plastic injection molding
for machines from 50ton to 2000ton; so when the time comes to make a
case, I know how to get a mold done on a budjet. But I am thinking of
staying away from plastic. The case now doesn't have a lot of water on
the other side. I think it now has to be treated more like an electrical
cabinet. We need to consider shorting events as the cases get smashed in
a wreck.



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## EVDL List (Jul 27, 2007)

I ran across these photos of how the killacycle pack is done
http://www.veva.bc.ca/features/killacycle/a123-killacycle.jpg

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## EVDL List (Jul 27, 2007)

[No message]


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## EVDL List (Jul 27, 2007)

Bill D wrote -


>A while ago I posted the following formula for sizing cables, bus bars, 
>etc.:
>
> >>>>> Wire sizing <<<<<
> It is all about heat transfer.

Snipped

> Gauge RMS Amps
> 10 120
> 8 170
> 6 280
> 4 530
> 2 700
> 1 800
> 1/0 1100
> 2/0 1300
> 3/0 1700
> 4/0 2100

In the link just put up showing the battery pack of the Killacycle it shows 
a copper bus bar. In the close-up it looks like a copper pipe that has been 
flattened to hold the cable, you can see the wall of the pipe at the top and 
the connectors have been spot welded to it. Is that what is used as a bus 
bar, a copper pipe? If so where does the copper pipe fall on the list of 
Gauge/RMS Amps above?

Rush
Tucson, AZ
2000 Insight, 66.7lmpg, #4965
www.ironandwood.org
www.Airphibian.com
www.TEVA2.com 

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## EVDL List (Jul 27, 2007)

When sizing buss bar to match a wire gauge, do you just need to 
match up the cross-sectional areas, or is there more to it than that? 
For example, circular 2/0 wire (.365-inch diameter) has about 1/10 of an 
inch of cross-sectional area. So would copper buss bar that was 1 inch 
by 1/8 inch, giving a cross-sectional area of 1/8 of a inch, be a 
more-than-sufficient match to 2/0 wire?

Thanks.

Bill Dennis

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## EVDL List (Jul 27, 2007)

There is one "bus bar" per end of the battery pack.

I calculate the wire size needed for the whole pack current and then 
stuff a bare end of that sized cable into a copper pipe and pound it 
flat. (No need to make this overly complicated when a simple solution will do.)

I trim the cable so that it extends about 2/3rds the way down the 
pipe. At the halfway point on the bare cable, I cut away about 1/3 of 
the strands. This makes a roughly tapered conductor that has a very 
roughly uniform current density. No need to carry the extra weight of 
a full cross-section the full length of the bus bar.

Here is a link that shows the cross sectional area of standard conductors:
http://wiki.xtronics.com/index.php/Wire-Gauge_Ampacity

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## EVDL List (Jul 27, 2007)

Thanks, that was just what I was looking for. The 2/0 is the range of
operation I want.
If I look at the CSA of 2/0 as .1045 sq inches I can see that .156x.750
is greater so I can shave down from my planned .187 x .750.
I will probably reduce the .750 dimension as well once I decide on the
hole size.


Don't panic, these will taper back to .156 x .250 or .375 as they move
away from the inter-connect area; to save weight. The cell that is
furthest from the main connection doesn't need all that busbar.
Interconnects will then probably be .125 x .750 and .156 x .750
depending on parallel vs serial connections.

I want power, but I don't want it to add too much weight. The .750 is
really for ability to have a hole and a bolt for the interconnects and
the .156 minimum thickness is for something to weld the tabs on to.


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