Battery interconnect sizing

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soyachips
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Battery interconnect sizing

Post by soyachips » Fri, 14 Jun 2019, 21:38

I’m planning on making battery interconnects out of aluminium flat bar but not sure what thickness to use. I’ve seen people use multiple thin interconnects stacked on top of each other so I’m assuming the interconnects between each cell need to carry the same amount of current as the cables going from the battery pack to the controller but thought I’d check. My controller is rated to 400 Amps for 30 sec or 150 Amps continuous. For the cables, I’m going to use 50mm² orange power cable.

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Re: Battery interconnect sizing

Post by brendon_m » Fri, 14 Jun 2019, 21:45

Yes the interconnects will be carrying full battery current.
Thin sheets stacked up are better for a few reasons but the main one for diyers is probably flexibility.
Any reason for aluminium and not copper? Copper is a much better conductor

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Re: Battery interconnect sizing

Post by jonescg » Fri, 14 Jun 2019, 21:47

soyachips wrote:
Fri, 14 Jun 2019, 21:38
For the cables, I’m going to use 50mm² orange power cable.
There's your answer. Aim to have the same amount of conductor throughout. If you use aluminium make sure the cross sectional area is at least twice as much.
I'd use 2 mm x 20 mm copper at a minimum. As Brendan says, stacks of thinner stuff is great for flex.
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Re: Battery interconnect sizing

Post by brendon_m » Fri, 14 Jun 2019, 22:00

Oh and depending on the width of the bars but around 2mm minimum thickness for copper and aluminium is 2/3 as good a conductor. If in doubt go thicker when it comes to wiring (and bus bars in this case). Also the bus bars will be sinking heat from the cells so more meat can help there too

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Re: Battery interconnect sizing

Post by soyachips » Fri, 14 Jun 2019, 22:01

I was thinking aluminium mainly due to availability and partially cost. If I go with copper it needs to be tinned I believe and I was having trouble finding lengths of tinned copper flat bar. I need to make the interconnects myself as my pack is a slightly odd arrangement of cells! If you know of any good suppliers of tinned copper flat bar that would be great. I think I read about someone getting things tinned so that could be an option but need to look into costs.

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Re: Battery interconnect sizing

Post by brendon_m » Fri, 14 Jun 2019, 22:08

Copper doesn't need to be tinned, that just helps with stopping corrosion.
Im just going to use copper pipe from bunnings, squashed flat and drilled. I'll be using dielectric grease and soft seal to stop corrosion

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Re: Battery interconnect sizing

Post by brendon_m » Fri, 14 Jun 2019, 22:12

Not my calculations but

My calcs yield, ito of number of an equivalent number of #12 solid wire:
1/2 copper, thin wall (red): 10 wires.
1/2 copper, thk (blue): 15 wires
3/4 copper, thin: 22 wires
3/32 x 3/4 busbar: 15 wires

Figger 30+ amps per wire (uninsulated, and proly a conservative number), and
wow.... 300 to 660+ amps!

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Re: Battery interconnect sizing

Post by soyachips » Fri, 14 Jun 2019, 22:20

Hmmm interesting. I’ve seen people using copper pipe before but thought the corrosion thing was an issue. Good to hear there’s a fix for that. So dielectric grease between each of the stacked sheets and then soft seal around the outside? Will you spray the soft seal on the interconnects before attaching to the cells or after?

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Re: Battery interconnect sizing

Post by antiscab » Fri, 14 Jun 2019, 22:23

out of curiosity, what connector spacing are you looking at?
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Re: Battery interconnect sizing

Post by brendon_m » Fri, 14 Jun 2019, 22:26

After, soft seal would get in the way or making good contact.
For full disclosure, I have just finished making up my bus bars and due to the way my pack will be and I'm going to double stack the bars I had to flatten 52x 300mm bars and drill 312 holes. I don't wish that on anyone.

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Re: Battery interconnect sizing

Post by soyachips » Fri, 14 Jun 2019, 22:37

Wow that is a lot of holes!!! Actually just did a quick calculation and mine will be around 200 holes! :shock:

Thanks for the quick replies, great info and I might end up going down this path.

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Re: Battery interconnect sizing

Post by soyachips » Sat, 15 Jun 2019, 02:48

antiscab wrote:
Fri, 14 Jun 2019, 22:23
out of curiosity, what connector spacing are you looking at?
That’s a great question! I may end up regretting this but I’ve ended up with 24 x 15Ah and 24 x 10Ah Headway cells that I’m putting together into a 24s2p kind of arrangement. Each 10Ah cell is “piggy backing” on a 15Ah cell using 3D printed brackets and because they’re shorter, the connectors at both ends will have steps in them. Here’s a photo of a partially completed pack to help explain.
D373F68E-2ABC-4757-BAB7-A7A33D15D122.jpeg
D373F68E-2ABC-4757-BAB7-A7A33D15D122.jpeg (1.46 MiB) Viewed 895 times

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Re: Battery interconnect sizing

Post by brendon_m » Sat, 15 Jun 2019, 05:36

Is the plan to have a square on the bottom that's 2 holes x2 holes and strips that bend up and down across the top that are 4x1?
Or maybe 4x1 strips along the bottom with 2x1s on top joining 2 10ah cells and separate one for 15ah cells?

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Re: Battery interconnect sizing

Post by poprock » Sat, 15 Jun 2019, 06:11

Re brendon m's comment on cu vs al, don't remember much from my '68 army apprenticeship but I do remember that the figure being taught then was 62%. Still learning from this forum for my third conversion.thanks.

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Re: Battery interconnect sizing

Post by soyachips » Sat, 15 Jun 2019, 06:14

poprock wrote:
Sat, 15 Jun 2019, 06:11
Re brendon m's comment on cu vs al, don't remember much from my '68 army apprenticeship but I do remember that the figure being taught then was 62%. Still learning from this forum for my third conversion.thanks.
I found this online:

Aluminum has 61 percent of the conductivity of copper, but has only 30 percent of the weight of copper. That means that a bare wire of aluminum weighs half as much as a bare wire of copper that has the same electrical resistance.

So going with aluminium is actually lighter :?

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Re: Battery interconnect sizing

Post by soyachips » Sat, 15 Jun 2019, 06:22

brendon_m wrote:
Sat, 15 Jun 2019, 05:36
Is the plan to have a square on the bottom that's 2 holes x2 holes and strips that bend up and down across the top that are 4x1?
Or maybe 4x1 strips along the bottom with 2x1s on top joining 2 10ah cells and separate one for 15ah cells?
I’m still working that out as well as some way to bend the metal accurately. Given the connectors have steps in them I’m thinking of doing them in one piece rather than multiple thinner pieces stacked together so I don’t have to do bends in different places for each level!

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Re: Battery interconnect sizing

Post by brendon_m » Sat, 15 Jun 2019, 07:23

soyachips wrote:
Sat, 15 Jun 2019, 06:14
Aluminum has 61 percent of the conductivity of copper
2/3 vs 61%.
Im happy with my memory from tafe.
soyachips wrote:
Sat, 15 Jun 2019, 06:14

So going with aluminium is actually lighter :?
But it will be harder to bend because it'll be thicker

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Re: Battery interconnect sizing

Post by Adverse Effects » Sat, 15 Jun 2019, 07:27

would this type of connector be better?
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Re: Battery interconnect sizing

Post by antiscab » Sat, 15 Jun 2019, 10:08

when I last dealt with a big headway build, I bought a sheet of 1mm thick copper (1200 x 2400) and had it water cut to size and hole spacing, then tin plated at a radiator place. Cost like $2k from memory to buy cut and tin the whole sheet. Unfortunately I had it so the cells were touching each other to save space. I didn't end up using the plates, and still have them.

Probably cheaper to get aluminium sheet laser cut and bent. Heaps of places that will source and do it for you from a diagram. Use some noalox to stop the aluminium from corroding at the connections.
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Re: Battery interconnect sizing

Post by soyachips » Mon, 17 Jun 2019, 06:30

Adverse Effects wrote:
Sat, 15 Jun 2019, 07:27
would this type of connector be better?
Thanks I may end up using something like this if I can’t get solid bars to work.

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Re: Battery interconnect sizing

Post by soyachips » Mon, 17 Jun 2019, 06:40

antiscab wrote:
Sat, 15 Jun 2019, 10:08
Use some noalox to stop the aluminium from corroding at the connections.
Thanks I’ve seen a lot of people using this. I’ve also seen people using Chemtools R006 which is a conductive carbon grease. Any thoughts on the pros and cons of both?

Shame you couldn’t use the plates you had made up! What was the configuration of the Headway pack and how did they perform?

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Re: Battery interconnect sizing

Post by soyachips » Mon, 17 Jun 2019, 06:45

brendon_m wrote:
Sat, 15 Jun 2019, 07:23
But it will be harder to bend because it'll be thicker
I’m going to try using 25mm x 3mm thick aluminium bar and just bought one of these vice brakes to do the bending. Will post the results here.

https://www.machineryhouse.com.au/V050

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Re: Battery interconnect sizing

Post by Richo » Mon, 17 Jun 2019, 13:25

Stuff the bending - I'd go with M4 Metal spacers NPB F-F and just bolt the different height busbars together.

What's the height difference?
So the short answer is NO but the long answer is YES.
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Re: Battery interconnect sizing

Post by Dan007a » Mon, 17 Jun 2019, 17:06

Have 25mmx4mm ali.
Fyi. Make sure your nuts are tight. Ali gets very very hot if loose.

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Re: Battery interconnect sizing

Post by soyachips » Mon, 17 Jun 2019, 18:14

Regarding full current going through the interconnects, I keep thinking about those battery packs made up of lots of 18650 laptop cells and wondering how they get away with using small/thin tabs between the cells? What am I missing?

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