Braided Copper Ground Straps.

Over the past three years we’ve paid careful attention to our battery connections. There’s a reason. Even in the days of lead acid Trojans, it was not unusual to blow the entire CORNER of your battery case off with an almost explosive event over a battery connection.

These aren’t terribly dangerous. But they’re not very convenient either. Kind of like a rifle shot going off behind your ear. And you are stranded. Of course, you also lose the expensive battery.

So we mind our connections. I really rather liked the little bent copper straps we got with our first few sets of cells. And the steel M8 bolts and lockwashers were a HUGE improvement over the slotted soft aluminum screws we got with our first set of Seiden LiFePo4 cells.

And they worked well enough. But there were some disadvantages that just kept growing on us. The most worrisome is the lock washers. The threaded holes in the cells ARE soft copper and aluminum. The M8 size and 1.25 mm thread are really a pretty strong size in that soft material. But you cannot overtighten these or you will strip them and have to retap with a larger tap and use a larger bolt.

More to the point, however you tighten them, if you check back in a month, they have all loosened up an 1/8 turn. Some a 1/4. Not good. As they loosen, resistance starts to build, corrosion occurs, and at some point you blow a connection from current flow. And quite possibly a cell this way.

The problem is that a car vibrates going down the road. Worse, the bolt, wssher, strap, and terminal are all different metals with different thermal expansion coefficients. And every time we press the accelerator, we heat them up. ANd every time we release the accelerator, we cool them down. Constant thermal cycling and constant vibration combine to gradually work even very good connections loose.

The other area of concern is current inflation. By that, I mean that three years ago 300 amps was quite a bit of current. Then 500 amps. Then 1000 amps. Now with teh Cobra, we’re doing 1200 and 1300 amps routinely. Now with the Escalaade, we’re talking about 2000 amps – or was it 3000 amps.

Those little bent copper straps are good up to a point. But in China, an electric car with a 10 kw motor is motoring. Here, we’re starting to look at 150 kw, 200kw and in teh case of the Elescalade, 400kw power plants. That’s a lot higher current than we were dealing with just a year or two ago.

Over time, we’ve come up with a terminal connection scheme that has worked EXTREVELY well for us. We no longer HAVE loose connections AT ALL. We can handle a lot higher currents. We no longer are “prying” against the terminal with every flex of our battery pack. The whole pack is “flatter” and in fact looks a lot better. So what did we do?

1. Braided copper grounding straps as straps. These are copper of 50 sq.mm cross section. But it is tinned to reduce corrosion and increase connectivity at the terminal surface. They flex in the BRAID. So the encased part at each end is held flat to the terminal. As the pack flexes, the braid takes up the flex. There is just more copper in these too so they can carry more current.

2. A little upgrade on the bolts to 18-8 stainless steel.

3. Nordlock Washers. http://www.nordlock.com. This Swedish company has invented a wedgelock washer that is very hard, and features a kind of reverse CAM between the two active pieces. Each piece bites into the adjoining hardware to grip it, and then to LOOSEN the bolt, you have to work against the cam direction between the two washers. This actually INCREASES pressure on the bolt. They just don’t back out. Not from thermal cycling. Not from vibration. They are ORDERS OF MAGNITUDE better at keeping those connections clean and tight than the lockwashers we are so accustomed to.

4. Zinc coating on Nordlock washers. In a situation of dissimilar metals, in the presence of an electrolyte, you get a bit of galvanic action that causes corrosion. This can be diminished using a bit of Zinc as a “sacrificial anode” to give up electrons. So we use the Zinc coated Nordlocks.

We were getting our braided straps from EVWorks in Australia. This has been an excellent source of a number of components for us over the years. They appear to be currently undergoing some changes internally. We’re not sure what all that’s about. But we recently ordered some straps for our 400 Ah cells and received the wrong ones. Their response was bizarre. They’ve offered to change the text on their web site. And they don’t have any for our 400 Ah cells and won’t EVER have any.

Worse, they have recently had some new shipping deal that sent the cost of these way over $5 a piece by the time they hit the U.S.

So we had to cast about China to find a source for braided tinned copper ground straps.

We found one. And they made us stome straps for our 400 Ah cells. The problem is, they like to sell them in quantities of ONE BRAZILLIAN at a time. Now we are never going to need a brazillian 400a cell straps. But we do use quite a few of the straps for 180Ah and 160 Ah and 100 Ah cells from CALB and Thundersky. These are 98 mm long with 8mm x 13mm holes on 70 mm centers. So we talked them into making us a couple of hundred 400Ah straps if we ordered ONE BRAZILLIAN 70 mm x 8 mm straps.

They agreed. And four or five THOUSAAND dollars later, we have our straps and a lifetime supply of 70mm straps as well. And we love them.

We mentioned this on the show, and had three immediate sales of EVTV viewers who have builds going who needed small quantities of these straps. So we put together a little kit with a strap, two of the 18-8 stainless bolts, and two of the Nordlock washers – for $7 plus shipping. Our first three sets averaged about $30 for a $2 box and the UPS ground charges. Way different from shipping from Australia or China.

So we’re going to offer these sets complete with the two bolts, two washers, and one strap at $7. You don’t have to chase all this down the way we have had to. And you’ll enjoy clean, tight, safe high current connections that actually make your pack look good as well.

This is one way to support EVTV and help us reduce the inventory on hand of ONE BRAZILLIAN 70mm braided copper straps.

There are also straps with holes on 60mm and 80 mm centers – the 8 mm holes of course. We can get those as well if anyone needs them.

Send me an email to jack@evtv.me with the number you want and your shipping address. I’ll respond with a Paypal invoice with the shipping calculated – but usually $25-$30 here in the U.S. Each “set” weighs 0.19 lbs if you are into calculating such things.

Regards;

Jack Rickard

http://EVTV.me

26 thoughts on “Braided Copper Ground Straps.”

  1. History Repeats it Self

    Once upon a time the epitome of personal transportation was the horse and buggy. This industry was well defined and proceeding along nicely. And then, along came the first attempts at a horseless carriage.

    Anyone with an ounce of common sense knew that a team of horses attached to a buggy was:
    1. More reliable
    2. Didn’t break down or have flat tires
    3. The owners did not have range anxiety
    4. You could get fuel for the horses anywhere
    5. No one laughed at you when you drove by

    But after a long and difficult struggle the horseless carriages evolved into what we now know as modern day vehicle.

    Once upon a time the epitome of personal transportation was the automobile. It was powered by an internal combustion engine. This industry was well defined and proceeding along nicely. And then, along came the first attempts at an electric vehicle.
    Anyone with an ounce of common sense knew that a modern automobile was:
    1. More reliable
    2. Didn’t break down as often
    3. The owners did not have range anxiety
    4. You could get fuel for the internal combustion engine anywhere.
    5. It had lots of creature comforts

    But after a long and difficult struggle the electric vehicle will evolved into what we will know in the future, as a modern day vehicle for personal transportation.

  2. That new source for braided straps is great! Even better is the fact that you are selling them. I went through the exercise of looking for a source in the states… best price I came up with was $18 each if lots of 100. Would you sell them outside of the kit you’ve announced? Asking because I’ve already purchased 90 NordLoc washers (based on your recommendation)and my CALB cells came with stainless steel bolts. At a small extra fee of course.

    The braided straps seem like an ideal solution to battery shifting within the box. Which reminds me, I’ve been wanting to ask about how you secure the cells within the battery boxes. I don’t recall seeing you strap any of the cells into ‘modules’ which my local vendor is strongly advising me to do. Do you strap the cells together?
    Thanks and as always, looking forward to your next show!
    PeterH

  3. We do not really restrain the cells very well. They are not inclined to move once you put them in a box and strap them together. They kind of wedge themselves in and you could roll the car a dozen times I don’t’ think they could work their way out.

    That DOES kind of imply a certain amount of cells. In narrow strings and so forth, we usually put a strip of plastic porch decking over the cells beneath a secured lid. This is made by Weyerhouser and is called Colorfast Decking but most of the hardware guys just call it C-deck. quite strong. Flexible, and non conducting of course.

    Jack Rickard

    1. I found 100-110 inch-pounds appropriate for the M8 size bolt. Good and tight but low enough not to strip out the aluminum terminal. I highly recommend torquing to a known value. That way you can more easily detect changes. Because what Brian says is enough Jack may come along and say is not enough. If both of you have same reference there wont be this business of all of the cells need another 1/8 turn.

  4. I always wondered if there was a specific torque applied to each terminal connection and if any retorque was done. Sounds like the perfect solution with the braided straps and zinc coated Nordlock washers. So do you plan on checking the torque on your battery connections later down the road; say in a year?
    Great info Jack!
    Thanks!

    Jim Holifield

  5. Jack
    Seems you have the perfect solution with zinc coated braided straps with the zinc coated Nordlock washers. I always wondered if you used a specific torque on the battery terminals and if you checked torque later after initial installation. It will be very interesting to see the long term life of your 400Ah battery pack connections with these Nordlock washers. Thanks for the great info.

    Jim Holifield

  6. I’m not a metallurgist but I own a Euro repair shop and we’re pretty anal about following torque specs.

    38 in-lbs (4.3 Nm) is pretty weak for an M8 bolt. For reference DIN8.8 grade M8 bolts (~5/16″) are torqued at 25 Nm (18.5 ft-lbs), and A2-70 stainless bolts (equivalent to 18-8 or 304 stainless) are torqued to 22 Nm.

    But then we’re not here to guess. The bolts are not the concern but the copper (soft) and aluminum (softer). We don’t really know what kind of alloys they use but better to be conservative.

    Here’s a useful torque calculator:
    http://www.futek.com/boltcalc.aspx?mode=american

    Given that the tensile strength of Cu is 50000 psi and Al is 32000 psi, and a bolt length of 12 mm, my calculations came up with 12-13 ft-lbs.

    The bolt length is a crucial factor. Please play around with it so we can finally have a reasonable answer.

  7. Jack,

    If you plan on providing straps long term you might consider getting straps without a hole and having a local punch press shop make a die to shear the end off and punch a hole in them so that the same raw stock can be made into several different length and hole configurations.

  8. Borrowed from EV Source’s website:

    Cell Terminals Specifications:

    Recommended Bolt Recommended Lug Torque (ft-lbs, Nm)
    40AH M6x16 – M6x20 1/4″ 7, 9
    70AH M6x16 – M6x20 1/4″ 7, 9
    100AH M8x16 – M8x20 5/16″ 15, 20
    130AH M8x16 – M8x20 5/16″ 15, 20
    180AH M8x16 – M8x20 5/16″ 15, 20
    200AH M8x16 – M8x20 5/16″ 15, 20
    400AH M14x16 – M14x20 9/16″ 45, 60

  9. The 38 inch-lbs was provided by CALB. I don’t “calculate” such things. We asked and asked and finally that’s what the reply was.

    The problem of course is the soft aluminum and copper, not the bolt.

    We do not torque them at all. We guess. And we use a kind of short 4 inch stub ratchet on it and don’t really allow longer wrenches when tightening. That keeps it fairly consistent I think.

    The 70mm straps are just like the EVWorks straps 50 mm.sq. The straps with the 15 mm hole in it are twice as thick at 100 sq.mm cross section but they carry a lot of current and they are wider as well. The 70mm are only 24mm wide and I would think they would be VERY stiff at 100sq.mm.

    If there is a case to be made, but every swag at this is another “several thousand” as they don’t really do a dozen for you. They want a brazillian per order.

    Jack

  10. Jim Holifield. NOthing is perfect. I’ve been working on the same iced tea recipe for 32 years. It keeps getting better.

    But yes, we have had very good luck with our current system – essentially corrosion free and we can go back six months later and the bolts are as tight as when we walked away after strapping them up. They lie flatter, give us more headroom over the cells and I think they look better too.

    But as we kind of came up with this combination, I may be a blittle biased. it is working for us.

    Jack Rickard

    1. Yes, I have experience with electrically conductive grease along with all things electrical. I have much knowledge about the exchange that has occurred abt the ampacity of certain conductors. See I was an IBEW electrician before becoming an Electrical Engineer. Of importance to note, all the ampacities listed in the NEC do not account for air cooling like you will get in a vehicle, furthermore it does not represent at what ampacity the conductor melts into. The NEC can just be thought of as a ‘safe’ practice as far as ampacaties are concerned. I have more info on my blog at http://engineergreen.blogspot.com

  11. Jack.
    The battery straps you are using have a cross section of 50 sq mm. If you look that up in the AWG Chart 50 sq mm is the equivalent of a 0/1 wire which has a cross section of 53.5 sq mm. and a maximum current rating of 170 amps.
    How do we get away with using them in an environment where we expect to draw 3 or 4 hundred amps?
    This mount of current even for 10 or 20 seconds will cause heating of the battery straps. Have you ever experienced this in any of your battery testing?
    Al.

  12. I think your new “green screen” enhancement is very good! I thought the virtual white board worked well too. You definitely move your video production up a notch. (I hope you had fun doing it too!)

  13. Al.

    Look it up yourself. You’ll find the table references a wire length, often 1000 feet. A cross section without length has no meaning at all.

    In the case of the straps, we are traveling precisely 98 mm. And we are doing it with easily TWICE the cross section of the copper straps provided by the manufacturer.

    How do YOU KNOW that it will cause heating of the battery straps? Have you ever measured this? Or are you typing yourself smart on my blog?

    Straps do heat and cool and thermal cycle with every acceleration but not to the degree you infer. This is the reason for the Nordlock washers. and they work quite well, even without graphite grease.

    JacK Rickard

  14. I’d like to add on to what Jack says if I may.

    You can increase the current rating if your conductor is not insulated, has multiple strands and has a larger surface area than internal volume.
    For round wire with insulation its around 100mm^2 700A becomes thermally stable at 105C

    Jacks straps have 1.5X the surface area compared to round. So easily over 1000A continuous and nothing like the fuse strips provided by other firms.

    With all that current I’m more worried about Jack pulling away at the lights and all the manhole covers getting sucked up and attaching themselves.
    They have been talking about Earths magnetic pole about to flip… It’s all down to which way Jacks driving his ElEscalade at the time.
    If he drives around a few blocks all the migratory birds will get completely lost.

    I’ve gotta get me one of these!
    http://dcairns.files.wordpress.com/2008/08/snub.jpg

  15. The only sure fire way to get an indestructible connection for a ground strap is to bad weld it to the frame. Then and only then will you have no more problems out of it. Now for connections to the battery, I would recommend cad welding them to the wire also, then bolting them to the respective batteries. It may cost you a little more up front but the piece of mind knowing its indestructible and will outlast your car should make you sleep at night. As well you should come on over to my blog at http://engineergreen.blogspot.com where we talk of all aspects of ‘green’ engineering, vehicles not left out.

Leave a Comment