Advice on homemade pack

[csheep]

Hi all,

I am building a 10s4p pack of samsung 25R cells for my BBS02 36v 25A peak 750W motor. The pack will be made of two 4p5s packs as shown in the picture below. I have connected the cells within the two subpacks, I am using some nickel strips 7mm x 0,15mm. I am still waiting on a 10S BMS from elifebike to go on.

I have two questions about the next steps so I am looking for some advice:

- my nickel strips are rated for 20A constant current but my BBS02 may peak at 25A. I know the 20A is a conservative rating but I don't know how "conservative" that is. Do you think I am safe with these nickel strips ? If not, changing the strips is not an option, they are the only ones avaialble for the kind of cell brackets I am using. So, if needed, would it be ok to just add a "second layer" of nickel strips over the ones I already welded to double the cross-section area of my strips? Last option for me would be to downgrade the BBS02 controller to 20A which should be enough for the way I plan to use it.

- I hesitate between two different builds to connect the two 4p5s packs together:

Option A: I build one big "whole" pack: connect a 10S JST-XH balancing wire to the two subpacks, tape the BMS, tape the two packs together and put the big package into some shrink tube

Option B: I build it as two separate 4p5s subpacks with their own 5S JST-XH balancing wire, their XT60 +/- connectors and their own "shrink tube" protection. Then I add a XT60 series connection, a 2x5S to 10S JST-XH adapter (if that exists??) and tape the BMS on top of the two subpacks.

Do you guys have any idea on the best way to go?

Anyway, I am all ears if you have any other general advice about going on with this build!

Thanks a lot!

Csheep

 

[riba2233]

Those nickel straps are not 20A rated, they might work at that current, but they are only 0.5 mm2 copper equivalent. You can weld additional nickel on series connection, or beef the up with some copper wire, 1mm2 (solid is ok) per nickel connection.

 

[csheep]

Hi Riba,

Thanks a lot for the reply. Why weld nickel strip only on series connection and not parallel connections?

I am probably mistaken but I thought that basically there would be about 25A through the whole "square" nickel strip welding the 4p cells together and 12,5A through each of the series connection building two 4p blocks together. With that in mind, I would think that more nickel would be needed on the 4p blocks rather than on the two series connection between each block?

Also, I had computed that my nickel strip would be 7*0,15 = 1mm2 copper equivalent instead of 0.5mm2 equivalent. Where did I go wrong?

Thanks!

 

[riba2233]

Current goes through series connections in pack. In your arrangement, there is some current through parallel connections, but nickel is enough for that.
You are doing it wrong because nickel has 4 times more resistance than copper. So one 7x0.15 mm nickel strip equals in resistance to 0.25 mm2 copper.

 

[tomjasz]

I'm a little confused by this. What width and thickness of nickel would be appropriate?

My welder came to 0.1x4mm nickel plated steel can I use it? Or should I write it off?

 

[csheep]

Hey Riba,

Thanks for the reply, much clearer now. One last thing troubling me... I am wondering whether I made a mistake or not by linking the 4p cells with a "square" nickel strip (picture 1) instead of a "straight" line where I would remove one of the link of the square strip (picture 2).

I can imagine the current going from one block to another with the "straight" ligne build but the "square" strip build confuses me: the current of each cell of the 4p block can flow in two directions which does not seem very logic to me... for example, the current of lower left cell can go to lower right cell AND upper left cell and at the same time, the current of lower right cell can go to lower left and upper right. I put that in picture 3 to make the idea clearer

So what is really going to happen physically with my current parallel connections?

Thanks!

 

[riba2233]

I'm a little confused by this. What width and thickness of nickel would be appropriate?

My welder came to 0.1x4mm nickel plated steel can I use it? Or should I write it off?

They are equal in resistance to 0.1 mm2 copper, so go figure They are ok for parallel connection, but not for series.

You need thickness 4 times bigger than copper if you are going to use nickel, but that's not always possible and that's why I recommend nickel and copper combination. You need area appropriate for your current, good rule is 1mm2 of copper per 10 A of constant current.

 

[riba2233]

Hey Riba,

Thanks for the reply, much clearer now. One last thing troubling me... I am wondering whether I made a mistake or not by linking the 4p cells with a "square" nickel strip (picture 1) instead of a "straight" line where I would remove one of the link of the square strip (picture 2).

I can imagine the current going from one block to another with the "straight" ligne build but the "square" strip build confuses me: the current of each cell of the 4p block can flow in two directions which does not seem very logic to me... for example, the current of lower left cell can go to lower right cell AND upper left cell and at the same time, the current of lower right cell can go to lower left and upper right. I put that in picture 3 to make the idea clearer

So what is really going to happen physically with my current parallel connections?

Thanks!

Think like this, current will always flow where there's least of resistance, shortest path. So yeah, only one parallel link is necessary. In your third picture, all current will flow only through vertical lines. But parallel connections are still needed to keep cells in balance.