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Battery Cables Wiring Harness
How to Make Battery Cables
This is my basic guide to making battery cables. You will need the following:
Adhesive heat shrink - $2-4 / ft
Battery lugs - $1-3 each
Welding cable - $1-3 / ft
Welding cable cutters - $22
Hex crimpers - $90 (and up) for hex crimpers, $18 for a hammer crimper
(See more on these below)
Heat gun - $30-60
Lots of EV and solar power suppliers sell the heat shrink, lugs, and cable. You might find them at your local hardware store, electronics store, or welding supply.
Cable sutters and heat gun are common hardware store items.
You can get the job done with cheaper tools, or things you already have. There are lots of ideas out on the web (or see the FAQ). I'm just going to cover the basics of how I make cables.
Adhesive heat shrink is like normal heat shrink, but it has glue on the inside that melts and makes a tight seal. You can get different colors to color code the cable ends.
The lug above is a QuickCable Magna lug. You could use cheaper copper lugs, but the Magnas are very high quality and tinned so they won't corrode. I'm using 2/0 cable with a matching lug and 3/4" heat shrink.
You'll also need a lug crimper. You can get away with a cheap hammer crimper, but a hex crimper is really the way to go. Mine is made by Caly, it was about $90. It will crimp from 8-4/0 gauge cables.
For a sense of scale, these are about 2 feet long and around 15 lb.
Hex crimpers make a very neat and strong hexagonal crimp. (This is the only way to make UL listed crimps with Magna lugs.)
The jaws rotate for different size cables. The sizes are marked in mm^2. Go here to convert from mm^2 to AWG. (2/0 is 70 mm^2).
You'll also need a heat gun. I have one made by Ryobi, and it works great. It helps to be able to adjust the temperature from 250-1000 deg F.
There is a little attachment at the end that reflects the hot air for doing heat shrink.
First thing to do is cut your cable to the right length. Remember, measure twice, cut once.
Next, you want to use the same cutters to trim away the insulation. Rotate the cable through the cutter until you slice through the insulation, then twist the cutters to pull the insulation off the end.
You could also use a utility knife, but I find the cutters are easier. Don't worry if you take out a few strands of wire, but make sure you don't take off more than a few.
There should be just enough bare cable to fit into the lug.
Now put the lug into the jaws of the hex crimper and get it lined up. Insert the cable, making sure all the strands go into the lug.
On these lugs, there is room for two crimps, so I make one near the open end and another further up.
Make the crimp. I find it is easiest to rest one arm of the crimper on a table then use my weight to push down the other handle.
It takes a lot of force. You are actually cold welding the cable inside the lug.
When the crimps are done, they should look like this.
Since the Magna lugs are so thick, some material squeezes out of the side. I file this off so that it doesn't stick out.
Next, cut a piece of heat shrink long enough to go over the lug and about 3/4" down the cable. Put the heat shrink over the end.
If you can, set the heat gun on a table pointed up. Insert the cable with the heat shrink in place. I put my heat gun at about 400-500 deg for big thick heat shrink.
Keep heating until you see the adhesive melt and squeeze out of the ends. Move the cable around so that adhesive evenly squeezes out of both ends all the way around.
When done, the lug end should look like this. Note the adhesive that has made a ring around the ends of the heat shrink.
That's it! You've just made a pro quality battery cable.
As an aside, this is what the lug looks like inside. On the left, I cut the lug at the crimp. On the right is a fresh lug. Notice how the crimped lug basically looks like a solid chunk of copper?
It's hard to see in the picture, but even with a magnifying glass you can't make out where the lug and the cable met. It's been totally welded into a solid chunk.
What is 2/0?
Wire size is listed in gauges (or AWG) from about 24 down to 6/0. Smaller AWG is larger wire, until you get to 0, then it goes 0, 00, 000, etc. "00" is also written 2/0, and pronounced "two ought". Another common size for EV cable in 450 A systems is 4 AWG, not to be confused with 4/0. Roughly speaking, going 6 larger in AWG equates to about twice the wire diameter...so 2/0 is about twice as thick as 4 AWG.
You can find a chart of wire gauges here.
My controller manual says I can use 4 AWG cable. Isn't 2/0 cable overkill?
Maybe, but it doesn't cost much more. The main thing is to read your controller manual and see what they recommend for the current you will be drawing. I've seen from 4 AWG to 2/0 AWG for pretty similar loads, so it's not totally clear what is best. 2/0 can't hurt...but...thicker cable is less flexible, so not quite as easy to work with.
However, every little place you can lower resistance gives you better efficiency. Besides, those big fat cables look awesome.
The connectors between my batteries are just thin strips of metal. What's the point of using heavy cable when these strips are so thin?
First, those stips might be thin, but they're also wide. So their cross-sectional area is still pretty large.
Second, resistance depends on the cross-sectional area and the length of the wire. Those littls strips are only a couple inches long each, whereas your battery cables could be several feet or more. You need thicker cable to run long distances.
Why don't you use solder?
You don't need solder if you make good crimps. The pressure of crimping actually fuses the copper together inside the lug.
There's nothing particularly wrong with solder, and if done well it's fine. But crimps make a better connection, both electrically and mechanically.
(Note: I have used solder on the battery connectors for my GBS cells. That's because they don't have closed ends like the Magna lugs, so I made crimps and then used solder just to seal off the end.)
Do I need adhesive heat shrink?
No, but it will keep moisture out and help to keep the copper from corroding.
Should I use an anti corrosive or anti oxidant paste?
Can't hurt, but it might not be necessary. These pastes are usually for joining different types of metal, like aluminum and copper. Battery cable is copper, and most lugs are also copper. I don't think you need it. If you're using aluminum, definitely use an anti corrosive paste.
There are lots of brands, just make sure you get one made for electrical connections (zinc based). Do not use "dielectric grease", as it is not conductive.
So what's wrong with a hammer crimper? Why should I spend $100 on a hex crimper?
Nothing wrong with a hammer crimper, but they don't make as good a connection. It's also harder to get the crimp just right, and you can actually blow through the lug if you hammer too hard. If you do want to use a hammer crimper, don't use a hammer. Put the crimper in a vise.
Every connection can add resistance to the system. Better crimps can make the whole system more efficient. If you're spending $1000 or more on batteries, you might as well spend another $80 and do the cables right.
All of this sounds really expensive for just one set of cables. Is there a cheap way to get the job done?
Sure, there are lots of alternatives. You might save a few dollars here or there. Try searching the web for ideas, just use your head. If your project is worth doing, it's worth doing right. If you've already invested thousands of dollars, don't skimp on a $20 tool.
The big one is the crimper, and a hammer crimper will get the job done. You could solder if you already have a torch, but my advice is to just spend the $18 on the hammer crimper (at the very least). For large cable connections, crimping makes a superior connection.
You can cut cable with any number of things, a hacksaw, a jigway, a Dremel. You could use your teeth if you have a good dental plan. My opinion - I've used my cable cutters enough to make it easily worth the $22 I spent on them. It's up to you.
You can use a lighter instead of a heat gun, it's just tricky. You have to be really careful that you don't melt right through the heat shrink (really a mess if it's adhesive).