Common pack design mistakes, how to avoid?

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spinningmagnets
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Re: Common pack design mistakes, how to avoid?

Post by spinningmagnets » Feb 06, 2017 8:41 am

This is a great find. How interesting that a possible solution was right under our noses as a component of solar PV panel installation.

6mm X 0.2mm ribbon is equal to 1.2mm square wire. And that makes it equal to 16/17 AWG gauge wire. Thats fatter than needed for the parallel connections, but for the series? If there is one strip per cell (using common design of plastic cell holders), then...17-ga would be adequate for 3A continuous and 19A temporary peak? (per cell in the P-groups).

The tin-plating is for outdoor corrosion resistance, and it is very thin. The IACS conductivity of tin is 15/100, so...worse than the pure nickel at 22/100, but again...if its very thin there wouldn't be a lot of waste heat. The copper core would act as a heat-sink (thicker and wider would be better, of course). As a reminder, the copper core IACS conductivity is 100/100, and resistance is near zero if thick enough. So, 5-times as conductive as pure nickel (500% better)

[edit: pure nickel has a poor conductivity of 22/100, and copper is 100/100, but when copper busses are only plated with nickel, with the nickel acting only as a corrosion-resistance layer, the end result is rated as roughly 80/100. I suspect zinc-coated copper-strips would have a very similar result, and would be cheaper, plus easy to make at home]

What width of ribbon do most plastic cell-holders allow? 8mm? 10mm? (edit, apparently, 7mm wide nickel ribbon is the common standard)

edit: A quick google shows solar panel tinned bus strip can be had as tin-plated brass or copper. The tin-plating is VERY thin, but...I wonder if it is "just thick enough" to make spot-welding the ribbon onto the cell-end easy (tin has high resistance). Pic from Ulbricht ("bus wire for crystalline silicon")

Image

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Re: Common pack design mistakes, how to avoid?

Post by parabellum » Feb 06, 2017 9:00 am

spinningmagnets wrote:
What width of ribbon do most plastic cell-holders allow? 8mm? 10mm?
Those I have purchased and apparently most popular modular interlocking holders have 8mm groove.

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Re: Common pack design mistakes, how to avoid?

Post by fourbanger » Feb 07, 2017 1:08 am

So uh, back to the basic principle.. This is for a cylindrical battery I'm wanting to build using 6p modules of 18650s cut to fit inside a pvc pipe.
13s6p_1.1.gif
First draft.
(133.49 KiB) Downloaded 20 times
The gray is nickel, the yellow is copper wire. You get the idea..
13s6p_1.2.gif
Second draft. After reading this thread.
(134.08 KiB) Downloaded 20 times
Apart from twice as much contact area for the serial connection, not a whole lot of difference, really.

Does this pass inspection?

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Re: Common pack design mistakes, how to avoid?

Post by aethyr » Apr 17, 2017 9:30 pm

Image

Can somebody point me to a link or explain the proper way of soldering copper wire on to the nickel strip as shown in the above pic? I notice that the solder joints are between cells to minimize heat transfer to the cells. Even doing that, doesn't heat still, if somewhat reduced, still transfer to the cells?

Or perhaps did you pre-solder all the copper wiring onto the strip and then spot weld the strip onto the cells?

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Re: Common pack design mistakes, how to avoid?

Post by madin88 » Apr 18, 2017 12:23 pm

aethyr wrote: Even doing that, doesn't heat still, if somewhat reduced, still transfer to the cells?
The heat that reaches the sensitive part of the cell is so little in this case, it isn't worth to think about it
From the welding, the "heat peak" is much higher as the steel get melted.
I also would say now that soldering directly on the cells (with professional tools for a very very short time) also doesn't harm, or at least not more than welding (especially on the can).

The main reason i solder between the cells is that direclty on the welds the solder doesn't hold or flow that good.
aethyr wrote: Or perhaps did you pre-solder all the copper wiring onto the strip and then spot weld the strip onto the cells?
the sheets i beef up after welding them onto the cells, and i work from one side to the other step by step.
The area on the wire and sheet i couat with tin before soldering them together.
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Re: Common pack design mistakes, how to avoid?

Post by aethyr » Apr 18, 2017 8:45 pm

madin88 wrote:
aethyr wrote: Even doing that, doesn't heat still, if somewhat reduced, still transfer to the cells?
The heat that reaches the sensitive part of the cell is so little in this case, it isn't worth to think about it
From the welding, the "heat peak" is much higher as the steel get melted.
I also would say now that soldering directly on the cells (with professional tools for a very very short time) also doesn't harm, or at least not more than welding (especially on the can).

The main reason i solder between the cells is that direclty on the welds the solder doesn't hold or flow that good.
aethyr wrote: Or perhaps did you pre-solder all the copper wiring onto the strip and then spot weld the strip onto the cells?
the sheets i beef up after welding them onto the cells, and i work from one side to the other step by step.
The area on the wire and sheet i couat with tin before soldering them together.
Thanks, I think that was your pic to begin with :)

I think I'll follow your approach of pre-tinning the strips. That way I don't have to heat the connection as much.

One more question: your copper bus wire - doesn't current still have to pass through the thin nickel strip to reach the copper wire, and so doesn't the strip remain a bottleneck for the current? The reason I ask is that I'm designing a high current pack and I want to ensure no bottlenecks.

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Re: Common pack design mistakes, how to avoid?

Post by fechter » Apr 19, 2017 8:09 am

aethyr wrote:
One more question: your copper bus wire - doesn't current still have to pass through the thin nickel strip to reach the copper wire, and so doesn't the strip remain a bottleneck for the current? The reason I ask is that I'm designing a high current pack and I want to ensure no bottlenecks.
Yes, but if the copper is right on top of the strip, the path distance through the nickel will be very short and not result in significant resistance or heating.


I've soldered directly to the ends of cells before and not had any issues. Soldering to the strips will heat the cells way less than direct soldering, but you need to pay special attention to the insulation under the strips. The fiber rings will prevent melt through.
"One test is worth a thousand opinions"

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Re: Common pack design mistakes, how to avoid?

Post by randyc1 » Apr 19, 2017 10:57 am

How would this rate for current flow, assuming connections can handle load.
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Re: Common pack design mistakes, how to avoid?

Post by parabellum » Apr 19, 2017 11:21 am

randyc1 wrote:How would this rate for current flow, assuming connections can handle load.
Balanced, from cell level stand point. Only limitation is Current handling capability of the conductors

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Re: Common pack design mistakes, how to avoid?

Post by randyc1 » Apr 19, 2017 12:13 pm

I mentioned "assuming connections can handle load".

What i want to know is how is Current flowing through the cells,... flowing relatively equal or is one getting much more current than the others ??
Like in shematics on page 1.

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Re: Common pack design mistakes, how to avoid?

Post by parabellum » Apr 19, 2017 1:18 pm

randyc1 wrote:I mentioned "assuming connections can handle load".

What i want to know is how is Current flowing through the cells,... flowing relatively equal or is one getting much more current than the others ??
Like in shematics on page 1.
Ok, balanced from P group standpoint, conductor resistance is equal for every cells current path, if conductor is same at any point. Better this time? :)

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Re: Common pack design mistakes, how to avoid?

Post by Allex » Apr 20, 2017 6:36 am

fourbanger wrote:So uh, back to the basic principle.. This is for a cylindrical battery I'm wanting to build using 6p modules of 18650s cut to fit inside a pvc pipe.
13s6p_1.1.gif
The gray is nickel, the yellow is copper wire. You get the idea..
13s6p_1.2.gif
Apart from twice as much contact area for the serial connection, not a whole lot of difference, really.

Does this pass inspection?
Yeah, second looks legit!

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Re: Common pack design mistakes, how to avoid?

Post by Allex » Apr 20, 2017 6:43 am

parabellum wrote:
randyc1 wrote:I mentioned "assuming connections can handle load".

What i want to know is how is Current flowing through the cells,... flowing relatively equal or is one getting much more current than the others ??
Like in shematics on page 1.
Ok, balanced from P group standpoint, conductor resistance is equal for every cells current path, if conductor is same at any point. Better this time? :)
Same what, length? If we consider thickness then the length is not that critical any more, right?

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Re: Common pack design mistakes, how to avoid?

Post by parabellum » Apr 20, 2017 8:22 am

Allex wrote:
Same what, length? If we consider thickness then the length is not that critical any more, right?
Same conductivity. (In simple: use same wire )
Example: If you take just 1S4P, for simplicity, as pictured and beef up the conductor on negative side, your current path resistance will differ for every cell in this parallel group.

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Re: Common pack design mistakes, how to avoid?

Post by randyc1 » Apr 20, 2017 10:18 am

Is this diagram , colors represent current and Heat.

In this diagram where the wires are Red will also augment their resistance correct ?
...Will their Higher resistance at red points change the way Current flows through the Pack ?

Will More current flow through batteries where heat is less ( not 10A on each cell) ??

EDIT: Maybe it all balances because where wire is Red on one end ,..it is Green on otber end ?
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Re: Common pack design mistakes, how to avoid?

Post by madin88 » Apr 20, 2017 10:51 am

The current flow through every single cell should be well balanced. However, what whould you use as conductors between the groups?
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Re: Common pack design mistakes, how to avoid?

Post by randyc1 » Apr 20, 2017 11:00 am

Lets say i had 10g copper wire conductor soldered the nickel strips.

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Re: Common pack design mistakes, how to avoid?

Post by fourbanger » May 13, 2017 10:38 am

Thank you, Allex.

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Re: Common pack design mistakes, how to avoid?

Post by grinser92 » Jun 16, 2017 1:28 pm

most of the pack designs i have seen so far connect some cells in parallel und then connect these "blocks" in series.

is there a reason not to connect some sells in series and the connect these "lines" in parallel?
this way you could check evere single cell on its own with a BMS.

i am absolutely new to electric bikes and right now only reading, thinking and some planning and just wondered if i am missing a point there

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Re: Common pack design mistakes, how to avoid?

Post by fechter » Jun 16, 2017 3:08 pm

grinser92 wrote:most of the pack designs i have seen so far connect some cells in parallel und then connect these "blocks" in series.

is there a reason not to connect some sells in series and the connect these "lines" in parallel?
this way you could check evere single cell on its own with a BMS.
In that configuration, each series string would need its own BMS. By placing cells in parallel first, you only need one BMS. Cells within a parallel group will inherently be balanced with each other.
"One test is worth a thousand opinions"

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Re: Common pack design mistakes, how to avoid?

Post by spinningmagnets » Aug 28, 2017 7:20 pm

Nothing wrong with spot-welding. Once the machine is dialed-in, any average garage builder can get consistent results. However, I had always felt in the past that soldering was a poor substitute. I now feel that...if the proper tools and techniques are used, soldering can be a very viable option (especially resistance soldering).

The bottom of an 18650 cell (negative anode) is the more sensitive end, and care should be taken to not damage that end with too much heat, regardless of the method used. Because of this, I now think that individual cell fuse-wire is not only useful, but it is (IMHO) the best connection method to the negative end. This includes spot-welded fuse-wire, or soldered fuse-wire.

The positive cathode can take quite a bit of heat without suffering any damage. Of course I do not recommend applying lots of heat, simply do the minimum necessary to make a solid connection. Copper ribbon is the best material for series connections, for carrying the main pack current. It can be found locally in every major city, and 0.25mm copper can easily be cut with stout scissors. If you live in a humid climate where you are concerned about corrosion, the copper can easily be nickel-plated in your garage.

Copper is very difficult to spot-weld with an expensive machine, and the common units cannot do it at all. However, if you experiment to verify the proper technique, soldering can reliably attach copper ribbon onto the positive cathode.

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Re: Common pack design mistakes, how to avoid?

Post by Swe » Dec 30, 2017 2:03 pm

Allex wrote:
Oct 24, 2016 5:54 pm

Image
How much difference is this "worst current share" regarding current draw? I guess it depends but just wondering if it is more theory than real world problem? I was thinking of using N.E.S.E. module (see link below), and they look like the "worst current share" in the picture. So how bad is it? Each cell peak current 8A with Sanyo 3500mAh. Say 4P.

The modules itself can handle 50A with no problem, but will I have problem ragarding current path through cells?

https://endless-sphere.com/forums/viewt ... 31&t=88051

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Re: Common pack design mistakes, how to avoid?

Post by brumbrum » Jan 02, 2018 7:56 am

Maybe ok for 30a draw, but you may find over time that the cell voltages begin to drift apart as some cells are taking the strain more than others.... In theory.

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Re: Common pack design mistakes, how to avoid?

Post by Swe » Jan 13, 2018 5:18 am

brumbrum wrote:
Jan 02, 2018 7:56 am
.... In theory.
This is what I was thinking about. Anyone have experience of this or is most of it in theory? Then ofcourse each pack and material would be different.
So the theory is that the extra cm of material resistance (compared to first cell) would be so high that first cell would take so much more current that it would decrease its life time?
Then of course if material is too thin there would be heat, but if material is enough it should only be material resistance?

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