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The Italian Job
(04-15-2018, 09:00 PM)jeremy777 Wrote:
(04-13-2018, 07:02 AM)Onizuka Wrote: Hello Guys, for my busbars I used 1.5mm copper wire that i found in Brico shop in my area, is quite cheap, a couple of euros for around 4-4.5m rolls. 
Jeremy, you made very nice analyse, what is your opinion regarding best design of busbars in order to have as much as same current drawn from each cell?

did you use 3 or 4 strands twisted? 1.5mm diameter is 1.767mm², 3 strands is 5.3mm², 4 strands is 7mm².

I will go into more detail in my Build Thread (coming soon...) but I summarise here:
In my 64p an 80p packs I have used a very different design to everyone else since I haven't used the plastic cell-holders, but the principles are applicable to any physical design. I started from the realisation that dividing the loop would reduce the currents present and therefore the voltage drops, and that multiple connections between packs takes this further. I also wanted a modular build so I could easily work on a smaller part of the pack. The 80p is arranged as 5 cards of 16 cells as 8 welded pairs (I kept the pairs from the laptop packs I disassembled) with 1mm wire (0.785mm²) connecting along the edge twisted into a connecting loop in the middle. The 4 pairs near the connecting loop connect directly to it, the 4 pairs at the ends have about 5cm of wire to the connection. The cards stand vertically in a slotted plank and are fixed by their attachment loops to 1cm wide *0.5mm thick copper strips with M3×4mm screws. The positive terminals of one row of cards are attached onto the copper strip just millimetres away from the negative terminals of the next row attached to the same strip. The cards are all built to have the same capacity so there should be very little current in the copper strips. In this way the issues of current gathering only apply to the end connections to the whole battery.

The 64p, build in progress, a positive end from the middle:

The 64p, Negative end connecting strip is wider. The connection point is the extra hole in the middle.

The 64p, view between rows.

I know it looks much less professional than yours, But... Since the central 4 cells connect almost directly to attachment loop, the only drop is to the outer 4 cells on each side, down about 6cm of 0.785mm², which gives about 5.5mV drop at 1A/cell. So half the battery is at 11mV higher voltage during heavy-ish discharge.

To get to something like this from where you are I would put as many connection points between neighbouring packs as I could. If you have 3 strands of 1.5mm copper wire as your bus bars then all the figures I made in the analysis before about the 80p pack would double. I would suggest to divide the current between 5 connection loops central to each 4 rows of cells. If the loop is in the middle of the pack then the difference generated between front and back would be removed too, but it becomes more fiddly to tighten the screws. Each loop carries 16 amps at 1A/cell, so smaller loops and screws can be used than for the 80-amp end connections. With 5 interconnects (that works better physically than 4 because your interconnects would come between cells instead of on top of them) you'd have only the resistance of 4cm of 5mm² wire creating a difference between the cells near the attachment loops and the ones further away. at 1 amp/cell that's 2 amps on 144µΩ, or 0.288mV. I would do it with strips of copper sheet, soldered to the bus-bars, drilled and tapped for the screws so I don't have to fiddle with nuts and arranged so the screws come in from the front (as you have them standing) ... but maybe you can come up with something that looks more professional?

Hi, I used 4x1.5mm solid copper wire, total ammount 80-85€ for 14pack 80P
(04-16-2018, 07:21 AM)Onizuka Wrote: Hi, I used 4x1.5mm solid copper wire, total ammount 80-85€ for 14pack 80P

Please help Jeremy out as he can't find the solid core. That's what he's been looking for and only able to find stranded in his area. Perhaps you could point him in the right direction since you are also in Italy.
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Of course, he can find by Brico, Leroy Merlin or on ebay:
(04-16-2018, 03:45 PM)Onizuka Wrote: Of course, he can find by Brico, Leroy Merlin or on ebay:

Thanks - I think the nearest Leroy Merlin to me is in Torino, about 1.5 hours drive... My folks go down there about twice a year to go to Ikea...

But as you see I'm not actually looking for fatter wire since I already have just 11mV difference across my packs Wink

I do have a third brick to build so maybe I'll look to do that one with the 1.5mm.

I'm working on my full build story (so far) will post it soon...

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