Copper bus bars vs cable

[BigVT]

Does it matter as long as they're properly sized?

And, is it important they're all the same length?

 

[John Frum]

Does it matter as long as they're properly sized?

And, is it important they're all the same length?

Ideally whichever way you go they should be the same dimensions so that they have the same resistance.
Busbars don't have much "give" to allow for cell expansion and contraction.
There is concern that as the cells expand the terminals will be ripped out.
Much discussion about "clamping" and "strapping" the cells, but no consensus.

 

[Steve_S]

Couple of things to ponder upon.
1- Bus bars are solid and no "connections" which have potential for failures.
2- Cables require a lug on each end which is mechanically crimped on and could introduce a fault. (resistance, arching and corrosion, etc)
* any mechanical connection is subject to faults.
3- The "Contact Surface" on the cell is what transfers the majority of the power NOT the bolt / screw. It is important to get as much "Face" contact as possible and Bus Bars accommodate this easiest.
4- LFP cells expand & contract slightly during charge & discharge unless there is a critical failure.
-- Bus Bars should be the same length, width and thickness and appropriately sized for the loads intended.
-- Cables from Cells to Battery Terminals "should" be the same length to maintain energy balance.
-- BMS Harness wires should also be the same length with identical ends (consistency is important).

 

[BigVT]

Thanks for the quick replies. I'm looking to make a 4s pack for a boat, and a thought the cables might better cope with minor movement. Good point re mechanical connections.

I got a peli type case that can fit 4 cells if I connect them as 2 cell facing each other and then the short side towards the other 2 cells (hope that makes sense). I'm a bit worried about securing the short side of the cells (the ones facing each other shouldn't be a problem), and was thinking cables might solve micro movements. Perhaps I'm overthinking this, but what about mixing bars for the cells facing each other and cables for the 'side connection'?

 

[mrdavvv]

For mobile, its better for the cells to be completely fixed & strapped to avoid any movements. The terminals shouldn't cope with any mechanical load (Only the possible expansion / contraption of cells).

Also as Steve mentions, the cables have a big failure point, wich is the lugs itself. If they are not properly crimped you can have a point of high resistance, wich could increase heat, wich increase resistance and so on. If you have DIY cables, its very easy to fail to crimp them properly, moreso without adequate tools. Also mixing cables and busbars IMO its a bad idea, as they will have different resistance that can unbalance the cells.

You should always aim for a simple as possible system, and there is not a simpler option than a busbar.

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[Ampster]

I make my own buss bars and prefer the surface area connection I get. I have a hydraulic crimper so I am no stranger to crimps.

 

[KBWaldron]

With regard to busbars and concern about expansion of cells being restricted, consider this:

Make your busbars from copper foil. Folding a 38 gauge foil (available on Amazon) in half 5, or 6 times results in a "bar" 3/16 or 3/8 inches thick (it'll actually be a tad thicker), with 32 or 64 plies. You'll need a good vise to achieve tight compression. Form it with a slight arch and attach to cells. Now you have a very flexible busbar. If you look at the picture of the BYD cell connectors attached you will see this is what they did, using aluminum foil.
You'll want to use a very sharp drill bit at very high speed using light pressure to get a clean hole drilled. Best bet is to clamp between two sacrificial boards to support the foil.

 

[Ampster]

With regard to busbars and concern about expansion of cells being restricted, consider this:
Make your busbars from copper foil. .........If you look at the picture of the BYD cell connectors attached you will see this is what they did, using aluminum foil.
..............

That is good advice for motive applications like EVs and boats. Those BYD cells came out of EVs and that is a good example of why they do that.
For my stationary application I am using 1/4" Aluminum bars but am allowing a 1/8" space between the cells for any expansion. I am not compressing them but because I live in earthquake prone California I am clamping them. I expect that will make the building inspector happy also.

For anybody who is concerned about swelling I can give you a real world example. It involves some Thunderskys that swelled up when the inverter low voltage disconnect did not work while I was on vacation. The pack was two rows of 24 each and the bottoms fanned out but the custom copper buss bars did not destroy the cells. I saved 36 of them that were not badly swollen or had contracted and sold them to a guy doing an EV conversion. We tested them and the remaining ones had 90 to 95% capacity. Perhaps a testament to Thundersky and certainly and endorsement for LFP chemistry as being forgiving. Your mileage may vary.

 

[Deleted member 4428]

You'll want to use a very sharp drill bit at very high speed using light pressure to get a clean hole drilled.

A paper punch does a pretty good job if it is the correct size.