Offroader said:
Thanks, so you understand my concern about the other side connecting to different cells. I drew out a lot of diagrams and also don't think it would matter. Each individual series groups have exactly the same length wire.
Yes, I intend to actually weld a very small nickel strip, maybe 3 mm wide between the parallel groups for balancing. I will then run the balance wires to the bms. The reason for a thin strip is in case of a short it will limit the current draw from the other cells in parallel.
I didn't see the parallel connection so was really scratching my head.
The resistance of your wire is probably much less than that of the nickel strip, so the wire lengths are not going to be very critical. Depending on the copper gauge and the nickel, we can do the math on it, but WAG the copper is like 10x less resistance than the nickel strip for a given length.
Keep in mind there is "ideal" and there is "good enough". With an odd configuration, achieving ideal is going to be very difficult. The basic idea is to have many series connections so the current gets spread out and not concentrated on a single conductor. At the pack ends, you need a single conductor, so the simplest approach is to just use heavy copper so the voltage drop is minimal and the cells all discharge equally. Where you can run into trouble is when using only nickel strips and making every series connection through a single conductor.
Two goals in the layout from an electrical standpoint are
1. make sure the conductors don't get hot.
2. try to equalize the drain between cells in a parallel group.
Equalizing the drain is not so important if your discharge rate is low. When running 2C or higher, it might matter.
Another goal is ease of construction. Large nickel sheets that cover all the cells in a group is good for this.
Durability and safety are also very important. Avoiding potential shorts caused by vibration or bike crashes is high on my list.
You've got to love massive buss bars 8)
