Braided Copper Ground Straps: How We Solved the Loose Connection Problem for Good

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 lock washers 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 over tighten 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, washer, 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 Escalade, 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 lock washers 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 some 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 THOUSAND 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.

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.

Jack Rickard