# Making flexible battery connection straps



## bcwrench (Apr 5, 2011)

I had searched through some threads regarding making custom length cell straps for cells.

Although I don't have an electric car I do have what suits me better, an electric motorcycle.

When upgrading a cell pack in one of these, it presents a far more complex issue than doing a car. The issue is - SPACE.

My cells came with the usual copper straps which had worked not to badly in the short time I had used the cells before a BMS took out some cells. Never again.

An off-the-shelf motorcycle frame goes through a lot more twisting and movement than a car does just because of its dynamics.

I decided I would try to make my own flexible braided straps but was sort of put off by the usual and probably the most common method of soldering the ends of the braid. As many have found because of the wicking nature of the braid, the finished product is just about as stiff as the copper bar.

Many say the curve in the bar is to allow for some movement of the cells. I won't argue that point but I believe it is to allow for expansion and contraction of the bar as it goes through thermal loading expecially during large current draws. 

My problems were also compounded by the fact that many of the straps that came with my cells were not long enough because of my mounting constraints.

I pondered the issue for some time almost giving into doing it with crimp connectors and a cable made from numerous equal lengths of smaller wire for flexibility. In my case 3-10ga wires would equal the 4ga I required. This would introduce possible connection issues down the road because of multiple connection interfaces.

One day I was walking through the plumbing section of a local hardware store and happened to glance some short connectors for connecting lengths of tubing together. The lights came on!

It seems that many are having a problem coming up with a connector for the braided cable. The next day I checked the dimensions of braided cable that where available. The common ones were 4ga and 2ga. The measurement of the straps are 7/8 X 1/16 and 7/8 X 3/32. I ordered two of the 4ga 24" long.

I measured the inside of one of the pipe connectors (for 1/2" tubing which is 0.625 OD) and the circumference comes to just a bit bigger (1.9635) than the circumference of the braid (1.875). SHAZAM!

I squashed one of the connectors and found I could fit two (2) of pieces of braid into the copper. This would give and equivalent 1ga connector for anyone who needed something that big.

After marking up one end I made some smooth vise jaw adapters. To make an even better connection I then put the completed ends under a hydraulic press and gave them 5 ton push. After that I took a heat gun and applied some solder through the open ends. I figure this should be even better than the commercially available braided connectors and I can make them any length I wish.

After I made my first strap and liked what I saw. All I have to do now is make up a jig out of wood to allow me to place a small bend in each strap and them drill the holes for the bolts.

The nice thing about using braid is that it puts out less EMI that solid bar or cable.

Below is some pictures of my project.


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## Ziggythewiz (May 16, 2010)

Do you know the cross-section of the plain copper straps and the braid? I've been wanting to make some, but haven't found the braid I like yet.

What's your total cost per connector?

I'm also considering drilling and then dipping the whole end in a tin pot, but maybe wicking would be too much of an issue there as well.


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## bcwrench (Apr 5, 2011)

The copper strips are small as my system uses 40AH cells. The strips are 0.875 X ~0.050 which gives ~0.04375 which is close to a 3 ga wire size. The braid is a bit difficult to measure for thickness but it looks to be 0.875 X ~.060 which gives ~0.0525. The catalog calls it 4ga and the AWG table lists 4ga wire at 0.03278 so I'd say they are not underrating them. VD tests across the original copper straps gave inconsequential values. 

Cost of the ground straps was ~$11 ea (X2) The bag of connectors $6. The usable length of the strap is 22.5" This should give me 6 straps with some left over. If I shorten my lengths slightly I'll do 7 per ground strap. The bag of connectors has 26 units, so that good for 13 straps so................

$28 for materials / 12 = $2.33/strap or @ 13 straps $2.15 ea.

Nord-Lock washers are ~$20 for 25/pr and I already have the correct length bolts. So if I add the washers that adds ~$2 to each strap. Still change from a 5er.

EVTV has them @ $7 ea and they come with Nord-Lock washers and appropriate sized bolts but not in my length for my bolt size. And then there's freight of $45 to Canada. I need 10 straps so far so that is 115 cocos or $11.5 ea. And that doesn't include the duties and exchage rate.

Bulk ground strapping is available here in 25 ft roles.

The thing I like is being able to solder the ends to the copper thus reducing corrosion intrusion and giving that little bit better electrical contact. I don't know if the commercial straps are using a coated steel or coated copper band and whether the bands are soldered as well. My cells are not in direct contact with the elements but a lot closer to them than in a car. On top of that the bike is outside all the time and the ocean is a 1/4 mile from my door.

Using the solder pot would definitely be a faster approach. You might have to do a few sacrificial tests to get the full penetration without wicking to far out. Mind you, if you just inserted the end until hole just disappears that should suffice. Then again if you can cover the whole connector end you could possibly end up with a better contact because the solder will conform to any irregularities between the connector end surface and the cell terminal surface.

It's funny, that when one gets involved with EVs there seems to be that ultimate search for the ideal, almost zero loss connection.


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## Ziggythewiz (May 16, 2010)

bcwrench said:


> It's funny, that when one gets involved with EVs there seems to be that ultimate search for the ideal, almost zero loss connection.


Along with the search for the cheapest X!  I already have 20 ft of copper left over from making busbars for floodies, so I just need to find straps I like and the whole solder bit.

I'm also using 40AH cells, so I need to settle down to what I really need instead of trying to nearly match the 2/0 I use elsewhere.


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## Woodsmith (Jun 5, 2008)

Could you reduce the solder wicking onto the braid by clamping the bit of the braid that you want to keep unsoldered with a large heat sink?

I was thinking maybe a block of steel or aluminium clamped on each side of the centre section of the braid and then the exposed ends dipped in solder.
Never tried it so no idea if it would work.


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## PStechPaul (May 1, 2012)

That sounds like a good idea, to keep the solder from wicking too far up the braid (is that what upbraiding is?)

It may be good to make a proper crimper which actually compresses the copper pipe fitting rather than just squashing it flat. You just need to have two pieces with a V-shaped notch that fits the pipe, and when pressed it exerts force from both sides to make a true gastight compression fitting. The flat shape is rather weak in the middle and cannot exert as much force on the braid, so it may not hold a low resistance connection, and may allow contaminants in which could degrade the mating surfaces. Adding some NoAlOx before crimping may also help. And possibly a dab of electrical grade silicone sealant to finish the job.


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## bcwrench (Apr 5, 2011)

Woodsmith,

Good suggestion.

I made another 9 straps on Monday and timed myself. It averaged 15 minutes a strap.

It installed the braided section between the smooth jaws I made and clamped them together and used 15" of 0.030 resin core solder to fill the insided of each end. the solder did not flow dowm into the open braid.

Brainstorming works.


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## bcwrench (Apr 5, 2011)

PStechPaul said:


> That sounds like a good idea, to keep the solder from wicking too far up the braid (is that what upbraiding is?)
> 
> It may be good to make a proper crimper which actually compresses the copper pipe fitting rather than just squashing it flat. You just need to have two pieces with a V-shaped notch that fits the pipe, and when pressed it exerts force from both sides to make a true gastight compression fitting. The flat shape is rather weak in the middle and cannot exert as much force on the braid, so it may not hold a low resistance connection, and may allow contaminants in which could degrade the mating surfaces. Adding some NoAlOx before crimping may also help. And possibly a dab of electrical grade silicone sealant to finish the job.


 
I don't really follw you here. The ends of the strap have to be flat to allow drilling the ends for bolts to attach them to the prismatics I have.

To create a tighter mechanical connection for the copper I placed each end in a hydraulic press and gave then a 5-ton push. It seemed to give a vert consistent flat surface when measured with calipers.

After flattening I filled the ends with solder. This will serve three functions. Creating a better electrical connection, filling voids to reduce the compressibilty of the end and to reduce the intrusion of contaminants.


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## Woodsmith (Jun 5, 2008)

bcwrench said:


> Woodsmith,
> 
> Good suggestion.
> 
> ...


Yay! I had an idea that worked!


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## PStechPaul (May 1, 2012)

bcwrench said:


> I don't really follw you here. The ends of the strap have to be flat to allow drilling the ends for bolts to attach them to the prismatics I have.
> 
> To create a tighter mechanical connection for the copper I placed each end in a hydraulic press and gave then a 5-ton push. It seemed to give a vert consistent flat surface when measured with calipers.
> 
> After flattening I filled the ends with solder. This will serve three functions. Creating a better electrical connection, filling voids to reduce the compressibilty of the end and to reduce the intrusion of contaminants.


I think you are correct. I was thinking about other crimp connectors where the barrel is only a part of the connector and the flat part is already formed. Since the braid extends through the flat part, as yours does, the bolted connection provides continuous compression and maintains a good connection. The solder does a good job of maintaining the low resistance and sealing out contaminants. Good looking work!


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