Pack Busbar Sizing

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necrogt4
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Pack Busbar Sizing

Post by necrogt4 »

Hi all,

I'm planning on creating a battery pack of 31s4p with 50Ah cells. The pack will regularly see 2-3c so let's say 500A with a 750A peak. I'd like to use aluminium due to the lower weight but I'd need to use ~double the size of coper from what I can gather.

My question is, if per square mm of aluminium can only carry ~0.8A (vs 1.2A for copper) I'd have to run a busbar of 25mm x 25mm which is way to big to connect cells together (the busbars of each parallel module will be laser welded). Is this math correct or am I way off?
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jonescg
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Re: Pack Busbar Sizing

Post by jonescg »

For 500 A continuous I'd be looking at minimum 95 mm2 cable and buslinks. How to you plan on putting them in parallel? Making sure the current is shared evenly across all 4 parallel cells is key.
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necrogt4
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Re: Pack Busbar Sizing

Post by necrogt4 »

Sounds like I'm going to need a meatier busbar. Moving to 25mm x ~4mm should cover 95mm2 but I'm assuming you mean copper?

I'm looking to run the cells in parallel modules like so (see picture). How does one ensure current is evenly shared?


Image
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brendon_m
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Re: Pack Busbar Sizing

Post by brendon_m »

necrogt4 wrote: Mon, 31 Aug 2020, 10:55 I'm looking to run the cells in parallel modules like so (see picture). How does one ensure current is evenly shared?
What you have there will work except for the first(last?) module.
You just need to make sure the current enters and leaves each module from opposite corners.
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jonescg
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Re: Pack Busbar Sizing

Post by jonescg »

Copper is superior to aluminium in all but weight. Corrosion resistance is a nice touch too. Nickel plated copper is best, if you can afford it.
Remember, the outside of these cells is continuous with the positive terminal, so make sure you have a layer of FR4 between them, and ensure they cannot get wet from condensing humidity.
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brendon_m
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Re: Pack Busbar Sizing

Post by brendon_m »

Personally I think copper is the better way to go, it may be heavier but you don't need as much so it ends up being roughly the same weight. But then copper bus bars end up smaller so easier to work with and package
necrogt4
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Re: Pack Busbar Sizing

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brendon_m wrote: Mon, 31 Aug 2020, 11:04 What you have there will work except for the first(last?) module.
You just need to make sure the current enters and leaves each module from opposite corners.
That's such a good (and obvious now that I look at it with fresh eyes) point!! Thanks.
jonescg wrote: Mon, 31 Aug 2020, 11:15 Copper is superior to aluminium in all but weight. Corrosion resistance is a nice touch too. Nickel plated copper is best, if you can afford it.
Remember, the outside of these cells is continuous with the positive terminal, so make sure you have a layer of FR4 between them, and ensure they cannot get wet from condensing humidity.
The batteries will be in a sealed enclosure so condensation "shouldn't" be an issue. Though I'm not quite sure what you mean in regards to FR4 though Chris? The busbar across the top of the cells will be welded on (as the cells have no terminal attachments). Can you give me an idea of how/where FR4 would be used?
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Re: Pack Busbar Sizing

Post by jonescg »

As a thin insulating layer between two cells. Just to prevent the cells from rubbing through the plastic wrap and touching.
Welding the conductors on you say - Ultrasonic welds? Interested to see how you plan on doing this. Arguably a solid approach if you have the right kit.
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necrogt4
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Re: Pack Busbar Sizing

Post by necrogt4 »

Laser welded actually. Well that's the plan at the moment.

Good point regarding the very very thin plastic wrap. I am intending on using carbon fiber walls for the enclosure, I could quite easily have some more cut to size to create dividers. Technically I'd only really need it between each module. I'll be using thermal pads underneath the cells as they're sitting on a chill plate so they won't be touching that.

Here's a screenshot of the design so far... I need to change the clamp down bars so they don't cover the vent in each battery and also make a bit more room for wiring. I should probably make a thread detailing the process, if anyone was interested?!

Image
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brendon_m
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Re: Pack Busbar Sizing

Post by brendon_m »

necrogt4 wrote: Tue, 01 Sep 2020, 09:29 I should probably make a thread detailing the process, if anyone was interested?!
You can count me for 1 as interested
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Re: Pack Busbar Sizing

Post by rhills »

I think a build thread would be a very useful community resource!

Re the Carbon Fibre walls, I don't know much about this stuff, but I would have assumed carbon fibre was conductive. Is that assumption right or wrong? Does it matter?
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necrogt4
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Re: Pack Busbar Sizing

Post by necrogt4 »

Ok, well I'd better get into gear and put some photos and words up... I've been collecting parts and saving for a house since last year. I've got the house now and plenty of time at home in Melbourne so things are moving along pretty quickly now.
rhills wrote: Tue, 01 Sep 2020, 11:18 Re the Carbon Fibre walls, I don't know much about this stuff, but I would have assumed carbon fibre was conductive. Is that assumption right or wrong? Does it matter?
A quick Google search does confirm carbon fiber is indeed conductive. I see now why perspex is used. I'll either have to insulate the carbon fiber with paint or use a different wall material. Though it's an ideal material in terms of strength and weight. In terms of dividers between cell modules it's most certainly not the right choice then...
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Re: Pack Busbar Sizing

Post by tonyw »

I would suggest that no thickness of paint that you can apply will last under vibration. Better go for a solid insulator.
cheers

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necrogt4
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Re: Pack Busbar Sizing

Post by necrogt4 »

For whoever is interested, I've started a build thread...

/viewtopic.php?f=33&t=6647
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Re: Pack Busbar Sizing

Post by starryone »

Hi all,
This topic seemed a possible subject area for my first post.
The cross sectional area of a bare Copper conductor (multi-strand cable/bar/rod), can be closely estimated by weighing a 112mm length (111.6 actual) of the bare conductor. The weight in grams, is it's area in sq mm.
This is based on Copper's density of 8.96g/cc.

Consider a sq cross section bar, 10 X 10mm and 100 mm long. It weighs 89.6g. A longer 111.6mm piece of this bar weighs 100g, which is it's cross sectional area.

I have no electric vehicle, but used this method to estimate capacity of some surplus battery straps I modified, to interconnect 16 Avass (1st lot) 460 Ah cells.
These were integrated into my Victron Multiplius/Venus GX/dual mppt/Batrium WM4 6kW shed system.

This hint may be useful in some other diy threads.
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