Battery interconnect sizing

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soyachips
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Re: Battery interconnect sizing

Post by soyachips » Mon, 17 Jun 2019, 18:15

Dan007a wrote:
Mon, 17 Jun 2019, 17:06
Have 25mmx4mm ali.
Fyi. Make sure your nuts are tight. Ali gets very very hot if loose.
Thanks that’s a very good tip! I guess that happens with all materials though right?

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Re: Battery interconnect sizing

Post by brendon_m » Mon, 17 Jun 2019, 18:17

A lot of the cells are in parallel so each cell only sees a small portion of the current.
Each of your cells will see 1/2 the total current but from 1 pair of cells to next will be the whole kabootle

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Re: Battery interconnect sizing

Post by soyachips » Mon, 17 Jun 2019, 19:06

Richo wrote:
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?
The height difference is 15mm but there is up to 2mm variation so keeping the bars flat would handle the variations better and be a lot less work!

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Re: Battery interconnect sizing

Post by soyachips » Mon, 17 Jun 2019, 19:06

brendon_m wrote:
Mon, 17 Jun 2019, 18:17
A lot of the cells are in parallel so each cell only sees a small portion of the current.
Each of your cells will see 1/2 the total current but from 1 pair of cells to next will be the whole kabootle
Got it, thanks for clarifying!

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Re: Battery interconnect sizing

Post by brendon_m » Mon, 17 Jun 2019, 19:40

soyachips wrote:
Mon, 17 Jun 2019, 19:06
brendon_m wrote:
Mon, 17 Jun 2019, 18:17
A lot of the cells are in parallel so each cell only sees a small portion of the current.
Each of your cells will see 1/2 the total current but from 1 pair of cells to next will be the whole kabootle
Got it, thanks for clarifying!
Just to clarify further it's not exactly half because you have different capacity cells so I should have said 60%ish and 40%ish

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Re: Battery interconnect sizing

Post by Bryce » Tue, 18 Jun 2019, 10:55

Hi there - I'd go copper. Easier to work with, good conductivity, few issues with dealing with surface corrosion. The weight saving isn't worth the effort - in particular the difficulties of dealing with aluminium surface corrosion creating high resistance joins.
I buy copper bar of an appropriate cross-sectional area, use a bar bender to make them, then polish the contact surfaces. (Oh, and cover the rest with heat shrink to reduce bare conductive paths!!) For between cell links, I just buy whatever the manufacturer recommends. I also have a bag of flexible braid links if I need to deal with small height imperfections.

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Re: Battery interconnect sizing

Post by Richo » Tue, 18 Jun 2019, 12:38

soyachips wrote:
Mon, 17 Jun 2019, 18:14
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?
Ahh they don't have much choice.
And heat is related to cross sectional area AND length.
If the lengths are short the loss is low.

With the height difference an M4 spacer could handle 150A.
So 2 should do it...

edit: add quote
So the short answer is NO but the long answer is YES.
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Re: Battery interconnect sizing

Post by Richo » Tue, 18 Jun 2019, 13:02

soyachips wrote:
Mon, 17 Jun 2019, 19:06
The height difference is 15mm but there is up to 2mm variation
You also have to take off the thickness of the busbar.
If you use 3mm then the difference would be 12mm.
If you put a Washer at each end then your closer to 10mm.
https://au.element14.com/ettinger/05-04 ... p/1466853?
An M4 x 10 Pan with a spring under it should fasten it ok.
https://www.altronics.com.au/p/h3310a-m ... olt-pk-25/

Normally a jet cutter charges for the time+material.
They should stock 3mm copper plate.
So send them the design and pickup the finished part.
Bolt it together and you're done...
So the short answer is NO but the long answer is YES.
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Re: Battery interconnect sizing

Post by jonescg » Wed, 19 Jun 2019, 09:33

Richo wrote:
Tue, 18 Jun 2019, 13:02
Normally a jet cutter charges for the time+material.
They should stock 3mm copper plate.
I actually find supply + cut cheaper than me supplying material. I get 2 mm copper cut up all the time - it works out to about 1.8 c per mm of cut.
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Re: Battery interconnect sizing

Post by soyachips » Wed, 19 Jun 2019, 20:11

Thanks for all the suggestions everyone.

@jonescg I had no idea you could get copper cut so cheap. That's like 1/2 the price I could find on eBay just for the material! Who's your supplier?

I actually jumped the gun the other night after deciding to go with aluminium and bought some spacers from here http://aluminumspacers.com/. They're plenty big enough to handle the current and will use 25mm x 3mm aluminium bar to join the cells together like below. I'm using 50mm² copper cable so the aluminium bar is 75mm² to allow for it being 61% as conductive as copper. I'll put some conductive paste between the cells, spacers and bar to get good connections and prevent corrosion and use spring washers under the bolts to hold it all tight.
Headway-interconnects.png
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Re: Battery interconnect sizing

Post by jonescg » Wed, 19 Jun 2019, 21:25

soyachips wrote:
Wed, 19 Jun 2019, 20:11
@jonescg I had no idea you could get copper cut so cheap. That's like 1/2 the price I could find on eBay just for the material! Who's your supplier?
Intracut here in Perth. Mind you, 1.8 c per mm means a part like this:
20180330_182447.jpg
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Cost about $9 each...
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Re: Battery interconnect sizing

Post by Richo » Thu, 20 Jun 2019, 12:36

For small jobs you just end up with the Minimum Order Value.
Usually $65-100 depending on where you go.
So the short answer is NO but the long answer is YES.
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Re: Battery interconnect sizing

Post by soyachips » Tue, 25 Jun 2019, 18:31

OK some things I’ve learnt while waiting for parts to arrive ... aluminium is 61% as conductive as copper but that’s pure aluminium which is quite soft so it’s alloyed with other elements to make it stronger. Most common aluminium is only around 50% as conductive as copper so as @jonescg said at the very beginning, I need to use double the cross section I’m using for copper so will probably bump up the aluminium bar to 25mm x 4mm.

I also thought about using copper bars with my aluminium spacers but due to galvanic corrosion this seems like a bad idea.

Lastly I was trying to work out if I should use Noalox or a conductive carbon grease and read somewhere that Noalox isn’t actually conductive which is weird because a lot of people use it. So does that mean it should be applied after the interconnects are installed as a protective layer? Otherwise if it’s used between the contact surfaces how does that work if it’s not conductive? Are there any issues with just using conductive carbon grease?

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Re: Battery interconnect sizing

Post by coulomb » Tue, 25 Jun 2019, 18:36

I believe it squeezes out and only fills the gaps to make the joint corrosion resistant. So you don't use too much of it. The conductivity relies on the metal to metal clean surfaces. The Noalox just tries to keep them clean.
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Re: Battery interconnect sizing

Post by Richo » Wed, 26 Jun 2019, 13:07

soyachips wrote:
Tue, 25 Jun 2019, 18:31
... I need to use double the cross section I’m using for copper
But so far your reference is your 50mm2 copper cable.
The current capability of a cable is relative to its insulation, temperature rise and how its mounted.

This is NOT the same as the busbar.
It doesn't have any insulation.
So the only factor is the temperature rise that can affect the cells its bolted to.
The aluminum doesn't care if its at 25Deg or 300Deg.

I think there is enough info now on how your going to build the plate to determine the thickness based on appropriate temperature rise.
So the short answer is NO but the long answer is YES.
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Re: Battery interconnect sizing

Post by brendon_m » Wed, 26 Jun 2019, 13:18

Richo wrote:
Wed, 26 Jun 2019, 13:07
The aluminum doesn't care if its at 25Deg or 300Deg.
True, but to heat aluminium to 300 degrees takes energy, and that energy is coming from the pack.
Although my experience of aluminium is you look at it with an angle grinder in your hand and it instantly jumps to 1000000°c

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Re: Battery interconnect sizing

Post by Richo » Wed, 26 Jun 2019, 13:27

:lol:

Yep it does
I made a small ali plate with the dremel and that was bad enough.

I agree phatter is better.
But I still think that there should be enough info here to actually work it out and not just rule of thumb it.
So the short answer is NO but the long answer is YES.
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Re: Battery interconnect sizing

Post by mikedufty » Thu, 27 Jun 2019, 10:56

Can confirm, I've got a nice blister on my thumb at the moment from trying to hold down a bit of 2mm thick aluminium to drill a 4mm hole through it.

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Re: Battery interconnect sizing

Post by jonescg » Fri, 12 Jul 2019, 14:12

http://www.albusindustries.com/wp-conte ... USBARS.pdf

A very interesting read on aluminium and copper, and the application of nickel plating to both.
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Re: Battery interconnect sizing

Post by Richo » Tue, 16 Jul 2019, 12:54

Hmmm nickel plating aluminium.
Don't think I've seen that.

I'd still just nickel plate copper by choice for a busbar.

Aren't the headway ends nickel plated steel?
So the short answer is NO but the long answer is YES.
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Re: Battery interconnect sizing

Post by jonescg » Tue, 16 Jul 2019, 13:39

Yeah, I'm getting some aluminium nickel plated as we speak - keen to see how it pans out. They use a sulfamate process which seems to make it stick quite well. If this is the case, then I could replace the 2 mm thick copper I use in my buslinks with 4 mm thick aluminium and have no fear of corrosion. That said, aluminium tends to expand and creep more than copper, so there's a risk of screws coming loose over time.
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Re: Battery interconnect sizing

Post by jonescg » Thu, 18 Jul 2019, 12:14

Well the nickel plated aluminium worked well - nice finish and it takes 60:40 solder readily (with a 100 W+ iron). No peeling and the bond is strong.
They have to clean it thoroughly, then acid clean, then apply a zinc surface before electroplating with a nickel sulfamate solution.

I wouldn't make a habit of soldering busbars for serious duty, but if you needed to you could.
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Re: Battery interconnect sizing

Post by Richo » Fri, 19 Jul 2019, 12:40

That's why I suggested before nickel plated parts as they can be soldered to.

Was it a local plating job?
So the short answer is NO but the long answer is YES.
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Re: Battery interconnect sizing

Post by jonescg » Fri, 19 Jul 2019, 12:49

Richo wrote:
Fri, 19 Jul 2019, 12:40
Was it a local plating job?
Yep - Ultrachrome in Osborne Park. They do silver plating as well, but I reckon nickel plating would be a little cheaper and just as good.
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