Advice on designing a triangle 18650 pack

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wer2000   1 µW

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Advice on designing a triangle 18650 pack

Post by wer2000 » May 13 2018 9:45pm

Ok so I've been looking at building a new battery pack for a long time now and I really want to make an 8p13s 20amp discharge pack for my ebike. The only problem is I'm having a tough time coming up with a good cell layout for the space that I have. I have watched a lot of triangle pack videos and a lot of them are very different from one another and I have heard a lot of bad things about most of them due to poor designs resulting in uneven series discharging. Micha tolls triangle pack build here (https://www.youtube.com/watch?v=clawhNsORts) has been recommended to me multiple times where he connects one row of cells to another by using "jumper leads". The problem, however, is that all of the jumpers used are different sizes from one another. example (https://imgur.com/a/Q7vYldv) or is it really a problem at all?

Now if I'm right here, different length wires will have different resistance values, so over time, some cells in parallel will have different charge rates, right? Or is the effect too small to even outlast the life of the cells anyway?

One idea I've thought of is just to connect one set of cells to another using all the same size jumpers with no nickel in between
(poor example: https://imgur.com/a/BbXhxCr) "all blue lines indicated equal sized silicon wire connections"
With this method, all the cells are connected by the same length and size of wire so there should be no noticeable difference in the resistance values. Will this work or should another method be attempted? Sorry if my explanation is poor, I just wanted to get some expert opinions or ideas on this idea as I want to do this right the first time. Thanks again.

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spinningmagnets   100 GW

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Re: Advice on designing a triangle 18650 pack

Post by spinningmagnets » May 14 2018 7:37am

Equal current sharing is good, but I am not too worried about being exactly precise at every stage. I have a few suggestions, some of which may prove helpful. For common frame of reference, this is a triangle shaped 13S pack, and all P-strings are connected in the normal fashion except ...between P-string #8 and #9, we are using blue wires to make "jumpers". The shorter jumpers have lower resistance, and the longer jumpers have higher resistance.

Image

Instead of having a series connector from each cell to one other cell on the neighboring P-string (making 8 of them)...Make a series connection onto the parallel bus between two cells in the P-string, which cuts the number of series connections in half while still providing an equal amount of resistance for each cell. Now, you'd only have four series connections between the problematic P-strings to co-ordinate, instead of eight.

I can only assume you don't want to overlap the series connections because you are using circular-cross-section wire as the jumpers, and the point where they overlap will cause a lump. I can imagine making the shorter jumpers longer, and just coil up the excess length? another method might be to construct cross-overs from two flat strips of copper sheetmetal, with heat-shrink insulation over them.

In the pic below, the builder was not using enough series connections. The pack shown is 4P, with one nickel strip connecting the P-strings (red arrow). This lead to the single nickel series connection getting hot, the pack providing fewer amps than the cells were capable of (voltage sag), and un-even current sharing (cells near the single series connection provided more current than the cells farther away).

In this pic, I was also suggesting that the series connections should be copper.
BatteryBusBars (1).jpg
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BatteryBusBars (1).jpg (49.16 KiB) Viewed 278 times

wer2000   1 µW

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Re: Advice on designing a triangle 18650 pack

Post by wer2000 » May 14 2018 7:59pm

Not sure why I did not think about that but yea the idea of using four wires instead of eight is a whole lot better. Since it is easier to go the wire jumper route, and it should be almost the same as the copper crossover, I think I am eventually going to go that route. As for the wires overlapping, not a huge deal and I can probably arrange them to prevent that as much as possible. Thanks a lot for the suggestion!

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