Scrap copper busbars for high current pack


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harrisonpatm

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Jan 5, 2022
Messages
106
I'm working on an EV conversion for a motorcycle and am putting together a 12p24s pack using Battery Hookup's 32650 LiFePO4 cells. I chose these over Lithium Ion for their longer life, higher discharge rates, and thermal stability; if I get in an accident I don't want to risk fireworks. One of the downsides of course is the heavier weight. An issue I face for the pack as a whole is that a motorcycle (or any other EV really) is going to need to discharge high current rates, which makes building the pack with nickel strips an issue. I wanted to share my solution, in case it can help anyone else.

My math is telling me that I should expect 100A discharges on averages, with peaks up to 200A. The cells in 12p can handle that. It was the series connections I was worried about, specifically on the 23 series connections, each of which would need to handle the whole 200 amps. I'm using straight lines of 12 cells for each P group, next to the following 12 cells, and so on. My nickel strips are rated for only about 5 amps each, so even using one strip between each cell, for 12 strips, that capacity is only 60amps. I would need to stack 4 strips between each cell, and that's not very feasible. Luckily I learned, both on this forum and other resources, about copper strips that you can get on AliExpress (like this) which can carry a lot more current. Expensive, though.

I found a giant sheet of copper roofing at my local scrapyard, which I love, got it for 50 bucks. Same amount of copper purchased new would be several hundred dollars. I also like scrapping and reusing rather than buying new (I'm sure others on the site can get behind that). I can also take all my cutoff and sell it back to them for 10 or 20 bucks down the road, or reuse the scraps for other projects.
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Used a section of my nickel strip to mark the width I would need, which was about 15.5"
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Cut that with an angle grinder to get a long sheet at 15.5" width
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Made a simple wooden jig to stick the long sheet in, and cut off the exposed length to get 15.5" strips at just under 2 inches
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Made sure they were still matching up to my stencil
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So far so good. Now's the part where I really regretted doing this. My plan was to spot weld the 2p nickel strips in place on the batteries, then go back and solder the copper to the nickel, since it's basically impossible to spot weld copper with homebrew equipment. So I would need holes to place solder. I wanted holes at each junction for both the p and s connections between cells, as well as one hold on top of each cell for solder. I figured I could relatively safely solder on top of the cells at this point, since I would actually just be soldering on the nickel, which would also be disappating heat to the copper laid on top. That said, I decided to set myself up to drill 58 holes on each copper strip. For 23 strips, plus extras. One at a time. Yikes! This is the only part that would make me not recommend this method to others. Proceed at your own risk.

Sacrificial 2p nickel to mark my holes
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Drilled one, then used it at a stencil for the rest of them. Voila!
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After drilling, I put them through a few passes on the wire brush on my bench grinder, which smoothed out the metal shards as welled as removed any tarnish or discoloration, of which there were plenty, what with it being used for roofing. Not perfect I know, but I kept checking the sheets to make sure the holes were still lining up where I wanted them, and they were. This should be a great alternative to solid or stranded copper busbars, because it needs to be relatively compact and fitting in a box. I won't have time to get to assembling the pack for a few days, but I will update how it goes.

Someone want to check my math? The copper is 1.5mm thick, and it's going the whole 15.5 inches across the series connection (394mm), giving me a cross section of 591mm2. It's also on top of those 2p nickel strips, also at 1.5mm thick. I know that other factors are at play, like the holes I drilled not perfectly evenly and the strength of the solder connections, and the fact that it's a long flat cross section rather than something round. But even if you just look at the 591mm2 cross section of copper, that should give me several hundred amps of carrying capacity in the copper alone. Well above my peak usage of 200amps.

Would love to hear some feedback. I havent attached anything yet, so there's time for me to go back to the drawing board. I would only cry a little bit from the time wasted drilling hundreds of holes...
 

Korishan

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Jan 7, 2017
Messages
6,878
Nice find! Lots of work too. But at least you can get it done exactly, or pretty close, to what you need for your particular build.
 

italianuser

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Feb 25, 2020
Messages
502
Looks great and there's lots of satisfaction there, too, well done! (y)
Finding scrap material is a great thing, in your case you're using it immediately, I'm often in the other scenario where I find the stuff and put it in storage --> creating that "what will you ever do with all of this old metal, let's just get rid of it!" coming from wife ehm LOL :)

Amp capacity seems good if I apply the 4A/mm2 rule which I use for cables (but I'm never sure when talking about flat busbars!), waiting too see photos of the final setup.
 

harrisonpatm

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Joined
Jan 5, 2022
Messages
106
Looks great and there's lots of satisfaction there, too, well done! (y)
Finding scrap material is a great thing, in your case you're using it immediately, I'm often in the other scenario where I find the stuff and put it in storage --> creating that "what will you ever do with all of this old metal, let's just get rid of it!" coming from wife ehm LOL :)

Amp capacity seems good if I apply the 4A/mm2 rule which I use for cables (but I'm never sure when talking about flat busbars!), waiting too see photos of the final setup.
Sure, I used this copper piece right away. You should see the rest of my basement and all the other scrap I'm hoarding.

Ok, 4amps per mm2? Assuming that's a good rule of thumb, when applied to the 590mm2 on my series connections, I get a capacity of 2360amps.

Which is ridiculous. There's no way it could carry that. But it should do 200 amps just fine.
 

italianuser

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Joined
Feb 25, 2020
Messages
502
Ok, 4amps per mm2? Assuming that's a good rule of thumb, when applied to the 590mm2 on my series connections, I get a capacity of 2360amps.

Which is ridiculous. There's no way it could carry that. But it should do 200 amps just fine.
That's why I said I'm not sure about flat busbars! :)

The "4A/mm2" rule gives 10.48A ampacity for a 13AWG wire (2.62mm2) and 42.2A for a 7AWG wire (10.5mm2), measurements which do make sense.
 

floydR

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Joined
Aug 23, 2017
Messages
1,463
50.8 x 1.5 = 76.2mm2 x 4 = 304.8A seems much more believable, should be able to handle 200A with ease.
101.6 x 1.5 =152.4mm2 x 4 =609.6A
later floyd
ps Great find btw
 

bbbbrass

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Jan 17, 2022
Messages
51
I just used the battery hookup "Superbeast" headway packs to convert my golf cart to 48V. Works awesome. What voltage are you planning for your motorcycle?
 

harrisonpatm

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Joined
Jan 5, 2022
Messages
106
72v, which will actually be 76v nominal from 24s lifepo4. If I don't like it and need faster, I could always upgrade later or on a different build, but this is my first really ambitious build and I'm figuring out a lot as I go.

I had thought about using the headway cells, such high discharge current, but they cost like twice as much per kwh compared to most lifepo4 options. But had I gotten them, I still would have likely used a similar busbar method as what I'm using now, seeing how much current this copper roofing could carry.
 

harrisonpatm

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Joined
Jan 5, 2022
Messages
106
The main positive and negative bus bars need heavy duty wires running off them that can also carry 200 amps. In addition, my pack geometry is forcing me to make a "jump" between B15 and B16 because they're not directly next to each other. Since I'm doing these off the batteries, I can use a blowtorch for soldering and not have to worry about the connections breaking off later.
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Using 10 AWG wire (from scrapyard), rated for 30 amps each, 11 lengths on each connection, giving me 330 amp capacity.
 
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