# 2/0 cable vs. stock battery connectors



## Yukon_Shane (Jul 15, 2010)

I recently noticed a modern LiFePO4 powered build that used welding cable to make the connections between cells rather than the connectors that typically come with these cells which got me wondering why more people don’t do this. 
I’m assuming that 2/0 cable has a higher current carrying capacity then the rinky-dink connectors that come with these batters and it’s probably a lot cheaper than the braded connectors that EVTV and EVSource sell ($9/per connection!)
Any experts out there on current carrying capacity that can enlighten me (us) on this? Would 2/0 cable carry current better than the stock connectors? How about the braded cables that EVTV promotes? Has anyone quantified the improvement that these braded cables offer over the stock connectors?


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## EVfun (Mar 14, 2010)

I didn't use the straps that came with the cells because you need to take off the heat shrink and clean between each layer on the ends to insure a good connection to each layer. One layer of the provided straps wouldn't be big enough but with a bad connection between the layers that might be all you effectively get.

I'm using the EV Works (Australia) straps. The area of the braid is approximately equal to 1/0 cable (50mm^2, 1/0 is 53.5mm^2.) They are more flexible than the provided straps. It would be hard for me to make cables out of welding cable that are only 3 inches long, there would be more terminal that cable!


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## tomofreno (Mar 3, 2009)

Seems like solutions looking for a problem. I used the stock straps, fasteners, and clamps. I only cleaned the bottom of the bottom one and applied a thin film of Noalox where it contacts cell terminals. Been using them 3 1/2 years, over 25k miles with no issues. They have never felt warm to the touch, including 25 minutes at a bit over 1C (180A). Peak current is typically < 450A. Don't know about higher currents.


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## EVfun (Mar 14, 2010)

Clearly your straps where clean enough between the layers to establish a good connection. I don't trust parts shipped across the ocean that much.


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## dougingraham (Jul 26, 2011)

Yukon_Shane said:


> I recently noticed a modern LiFePO4 powered build that used welding cable to make the connections between cells rather than the connectors that typically come with these cells which got me wondering why more people don’t do this.
> I’m assuming that 2/0 cable has a higher current carrying capacity then the rinky-dink connectors that come with these batters and it’s probably a lot cheaper than the braded connectors that EVTV and EVSource sell ($9/per connection!)
> Any experts out there on current carrying capacity that can enlighten me (us) on this? Would 2/0 cable carry current better than the stock connectors? How about the braded cables that EVTV promotes? Has anyone quantified the improvement that these braded cables offer over the stock connectors?


The real question is how good is good enough. I have the GBS 100AH cells in my EV and during construction I measured the effective resistance of the supplied straps (tin or nickel plated.) For 1, 2, 3, and 4 straps measured from post to post (under the strap) at 30 amps I saw a voltage drop of:

1) 0.003170 volt 0.0001057 ohms.
2) 0.002360 volt 0.0000787 ohms.
3) 0.001778 volt 0.0000593 ohms.
4) 0.001227 volt 0.0000409 ohms.

You will note that it is not a linear drop in resistance and part of this is because of the interstrap resistance and part of this is due to the fact that there is a fixed resistance between the terminal and the first strap that doesn't improve by adding more straps. So back to my question of how much is good enough. I am using a Soliton 1 so 1000 amps is going to be my max current and that for not very long. How much voltage drop at 1000 amps and how much heat would be generated in the straps?

1) 0.1057 volts 105.7 watts
2) 0.0787 volts 78.70 watts
3) 0.0593 volts 59.30 watts
4) 0.0409 volts 40.90 watts

This is per cell so as you can see even with four straps there is quite a lot of waste heat at 1000 amps. On my 51 cell pack I see 2086 watts of waste heat at 1000 amps with four straps. The lost voltage due to this is 2.09 volts which is dwarfed by the battery sag (40 volts) under that kind of load. That is the extreme case and since you only ever hold that during hard acceleration I have found that with four straps I can't even feel any warmth in the straps. What about a cruising current of 200 amps?

1) 0.0211 volts 4.228 watts
2) 0.0157 volts 3.148 watts
3) 0.0119 volts 2.372 watts
4) 0.0082 volts 1.636 watts

So at typical cruising speeds the total loss at 200 amps across all 51 cells is 83 watts or around which is about 0.25% of the energy consumed. With one strap it would have been 0.66% of the energy used.

Could I have gotten away with one strap? Maybe. The problem would have been thermal runaway. The resistance of the strap would increase as the temperature goes up causing even more heat and more resistance. Hopefully everyone has seen pictures of what happens to posts and straps when this happens.

Are the braided straps better? Most likely yes in a couple of ways. Are they half the resistance? Probably not. The terminal/strap interface is still going to be an issue. Would making straps with lugs and 2/0 cable be better? Maybe a tiny bit which just is not going to matter. Cost of lugs and cable in addition to the time spent, I would rather use the braided straps.

At typical cruising currents none of this makes much difference.


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## Ziggythewiz (May 16, 2010)

They'd nearly have to be dipped in mud before shrinking together to have a bad enough connection for 3 in strips to not touch each other much.


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## PorscheFan5 (Mar 24, 2015)

dougingraham said:


> The real question is how good is good enough. I have the GBS 100AH cells in my EV and during construction I measured the effective resistance of the supplied straps (tin or nickel plated.) For 1, 2, 3, and 4 straps measured from post to post (under the strap) at 30 amps I saw a voltage drop of:
> 
> 1) 0.003170 volt 0.0001057 ohms.
> 2) 0.002360 volt 0.0000787 ohms.
> ...


Doug,

How about solid Copper Bar cut into bus bars? I intend on using 1/8" thick and 1" wide solid copper bar and cust the bus bars from it and drill 5/16 holes to connect my 45 180AH batteries.

Let me know what you think?


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## PorscheFan5 (Mar 24, 2015)

I would also put a thin layer of Vaseline (or some kind of dielectric contact gel) to keep the oxidation away from the connection surface of the bare copper...


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## PStechPaul (May 1, 2012)

Multiple layers of thin copper sheet (roof flashing) may be easier to work with and cheaper, and it provides flexibility as well as better cooling and lower resistance if you leave some air space between layers. I agree that some grease is good to keep the contact surfaces from oxidizing, but even better is a light silver plating which can be added using "Cool-Amp":

http://www.ebay.com/itm/Cool-Amp-Si...-Jar-1233-500-Plate-electronics-/161755518590









http://cool-amp.com/


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## favguy (May 2, 2008)

Porschefan5,

I made up my own connections out of solid copper bus bar. They are working well in my case so far. Can be seen in the pictures on this page:

http://www.diyelectriccar.com/forums/showthread.php/my-project-favelec-58958p14.html

Regarding Vaseline, I'll stand corrected, but I'm pretty sure this is an insulator!


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## favguy (May 2, 2008)

Just had a look at that cool-amp product, looks excellent


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## kennybobby (Aug 10, 2012)

You use silicone dielectric grease on spark plug boots because it is an insulating compound good for high voltage isolation.

You would want to use a conductive grease on battery terminals to prevent oxidation and maintain a low-resistance junction.

To have happy life--no get them confused and use in wrong place.


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## PorscheFan5 (Mar 24, 2015)

Nice work Fav!

What size bar did you use? Thickness, Width?
Also what is the nominal and max current you expect in your system?

Want to understand if my 1/8" thick 1" wide Bar is going to suffice for my 900Amp max system.

If Vaseline is not correct what did you use to keep oxidation/corrosion away? I see you have not plated the copper bars.


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## PorscheFan5 (Mar 24, 2015)

Thanks Kenny. Will do...

Fav,

Will look into Cool-Amp. BTW, what thickness Cu bar did you use?


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## PStechPaul (May 1, 2012)

The contact between surfaces of bus bars is actually hundreds or thousands of points of roughness that, with adequate tightening and pressure, cut through the grease, so its lack of conductivity is immaterial. It may be good to polish the mating surfaces with a wire brush so that the scratches are at right angles, and an effective grid of contact points is formed. For relatively thin and flexible bus bars, it may be desirable to use a thick, heavy, hardened washer to provide pressure distributed more evenly away from the bolt. Sometimes a Belleville washer is used for this purpose.

Here is more information:
http://www.csanyigroup.com/bus-joint-fundamentals
http://www.usbr.gov/power/data/fist/fist3_3/vol3-3.pdf
http://www.stormcopper.com/design/ampacity-tables.htm
http://www.copper.org/applications/electrical/busbar/busbar_ampacities.html
http://www.copperalliance.org.uk/do.../copper-for-busbars-all-sections.pdf?sfvrsn=2

I have considered using a thin piece of copper screen squeezed between mating surfaces of busbars to provide many points of connection. It's available from McMaster:
http://www.mcmaster.com/#9224t87/=xyucj1

There was a company that made thin conductive pads with tiny metal points embedded that were designed for optimal bus bar connection, but I'm not sure they are still around.

1/8" x 1" bus bar is good for about 250 amps continuous, and should be OK for 900 amps at about 10% duty cycle maybe 10 seconds maximum.

Here is a chart I made for bus bar and wire ampacity based on cross sectional area as well as surface area (which takes into account the better cooling of a wide, thin bus bar in free air):
http://enginuitysystems.com/files/WireSize.pdf
http://enginuitysystems.com/files/WireSize.xls


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## PorscheFan5 (Mar 24, 2015)

Excellent info. Thanks Paul.

I just changed my order from 1/8"x1" to 3/16"x1". Now I will have a Ampacity of ~340A at 30C. This is ~520A at 60C. I think that should be quite satisfactory, eh?

That takes care of it. Now to cut all the pieces, drill and prepare when I get the bars. Will investigate the conductive grease to use.


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## dougingraham (Jul 26, 2011)

PorscheFan5 said:


> Doug,
> 
> How about solid Copper Bar cut into bus bars? I intend on using 1/8" thick and 1" wide solid copper bar and cust the bus bars from it and drill 5/16 holes to connect my 45 180AH batteries.
> 
> Let me know what you think?


3/16" x 1" = 121 sq mm
4/0 cable has a cross section of 107 sq mm
3/16" x 3/4" = 90.7 sq mm
3/0 cable has a cross section of 85 sq mm
1/8" x 1" = 80.6 sq mm
2/0 cable has a cross section of 67.4 sq mm
1/8" x 3/4" = 60.5 sq mm
1/0 cable has a cross section of 53.5 sq mm
1 ga cable has a cross section of 42.4 sq mm
1/8" x 1/2" = 40.3 sq mm

I think you would be better off using the tin plated copper braid because it allows for some misalignment of the cells. And cells will not be perfectly aligned when the car is going over rough road. As was mentioned, thin plates of copper stacked to provide enough capacity would also allow for some flexing. You also have problems with dissimilar materials when you place copper and aluminum together. You are going to want to use an anti oxidation grease of some kind to keep out the air. And while these greases are not conductive that does not matter because the only place where conduction takes place is where the parts are in contact. The presence or absence of the grease makes no difference to the conductivity in the short term, but months or years down the line you are going to wish you had applied it. Plating the copper bars or plates with tin or nickel will prevent this as well.

Lastly you will want to use a lock washer of some kind to keep the bolt from backing out. It only takes one occurrence of this to ruin your day. You can lose a cell from heating a terminal to failure if a bolt loosens even just a little. My choice for lock washer is the NordLock washers. The zinc plated ones are probably the best because the zinc will corrode before the copper. I have not tried the new conical ones but they should be even better for this application because they provide some spring forcein addition to the cam action.

You can make copper bus bars work but I think you will spend a lot of time making them that would be better spent on other aspects of your EV build. You might even be better off making the bus bars out of aluminum since the battery terminals are aluminum and there will be no dissimilar metals problems. Aluminum bus bars with aluminum bolts and stainless NordLocks might be a better overall solution. And aluminum is lighter and less expensive than copper. You would need to make them a little thicker than copper.

Best Wishes!


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## favguy (May 2, 2008)

Hi PorscheFan5,

Sorry for delay in replying, been away. Personally I used 20mm x 3mm bar. My system is only 500A max with cruising current usually between 100A to 200A.

I polished both surfaces to bright copper prior to bolting, (the terminals are also copper on my cells), didn't use anything else based on the logic that the surfaces in contact tightly clamped together (of the same metal) won't tarnish and will remain low resistance, although the copper in general will tarnish slightly. My cells are completely insulated from any weather effects though being either inside the car or closed in a weatherproof box in the case of the front set. 

I used to work with distribution boxes in substations years ago where bare copper bus bars had been bolted together decades earlier and when we dismantled them despite the copper in general having darkened with age and tarnish, the clamped mating surfaces were always still bright on dis-assembly, hence I wasn't too concerned. Obviously it's a different matter if you have aluminium terminals meeting copper...


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## mizlplix (May 1, 2011)

*The following is my own opinion and may not agree with anyone else. I have found it to work well for me in the 15 years I have owned electric cars as well as the total 50+ years I have worked around electric powered equipment:
__________________________________________________
*
1-I would never use any type of solid connector between two lithium cells. The small vibrations and stresses would just add to the wear and tear of the terminals and possibly cause loosening and internal damage.

2-I never found the need to use giant connectors between cells as those connectors do not seem to run hot like motor leads do. 

3-I use the manufacturer's supplied multi-ply straps. Only a light surface cleaning was necessary. After the terminal bolt was torqued, I lightly sprayed a mist of clear laquer over each bolt/strap to exclude most oxygen from adding to the oxydation over time. If a complete disassembly and cleaning was necessary on these, wouldn't it be needed too on the woven straps? (and how you gonna do that?)

4-I make my own woven/flat straps where some cells are too far apart to use the factory straps. McMaster/Carr sells the 2/0 silver plated woven straps by the yard. I use small pieces of 1/2" copper tubing slipped over the ends, pressed flat in a vice, then soldered solid. I then drill my 5/16" holes and clean-install. 

5- I use those two piece locking/ratcheting lock washers on all cell bolts.
They are worth the cost. (Nordlock)

I have NEVER had local terminal heating, corrosion or loosening trouble. (After I switched to lithium cells, that is.....lead/acid batteries are a whole 'nother subject as most of the Golf cart battery terminals are rated at 75 AMps and will melt and cause problems when used heavily) Go here: http://www.diyelectriccar.com/forums/showthread.php?t=61014 to follow a friend's adventure.

Miz


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## PorscheFan5 (Mar 24, 2015)

As mentioned above I am using 3/16" x 1" solid copper bar. I have cut 3" pieces and drilled 21/64" sized holes (one step bigger than the M8 bolts need) about 7.15" apart. I know making the odd length connector will be a bit of a challenge but I will manage. Will use some kind of spring washer or even Nordlocks and will Noalox Anti-oxidant compound between the connections. The torque will squeeze out the compound from between the contact surfaces but the excess will keep the oxidation away from the surfaces open to air.


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## PStechPaul (May 1, 2012)

I tried to find any article suggesting problems with NoAlOx on copper, and it seems like it's OK. 

http://lists.contesting.com/_towertalk/1996-11/msg00118.html
http://www.garagejournal.com/forum/archive/index.php/t-187457.html
http://www.eng-tips.com/viewthread.cfm?qid=303652

But here is an article suggesting something else that might be better:

http://sw-em.com/anti_corrosive_paste.htm


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