Wire gauge and amps, series vs parallel

[john61ct]

Relevant background on where I'm coming from: I have only used large cells with post or bolt terminals, connected by wires with crimped ring terminals, each battery terminal only connected to one other terminal, same wire gauge for serial vs parallel.

So I really have a very hard time visualising the current flow through these connection "strips, grids and plates" layouts, where the same terminals are "communicating" with multiple other terminals, both serial and parallel at the same time.

So, to clarify my understanding, what about this "hybrid" 10P14S layout / design as an example:

14 separate sub-pack batteries

each a separate "box" using the welded grid/plate technology usual on this forum internally,

only **one big parallel group** of 10 cells each to get to the 10P

each group terminated from the grid/plate internally, to a [-/+] pair of threaded posts to the outside.

These "group boxes" are then wired in series to 14S

using insulated stranded heavy copper wire, say #2 gauge, to carry 100A bursts, 50A continuous.

Now, my puzzled question:

are the "intra parallel" connections **within** each of those 14 separate sub-pack batteries

truly **such** low current flow?

Take that 50A continuous "through" the 15 serial wires. Given the 10P group of say 2000mAh cells, each cell is getting an evenly divided 2.5C rate (5 amps) current flow through each cell-terminal-to-strip junction?

And so, that is what makes using only 0.15 nickel strips OK? really OK?

 

[DogDipstick]

Calculate it. Very well known and defined eq. to use.

 

[john61ct]

So should I infer that you are saying, my understanding of the overall concepts are correct?

The 10P14S design itself is sound, and when pack-level (serial) current is 100/50A, the parallel connections need sizing to handle only 10 peak / 5 continuous amps ?

 

[DanGT86]

Your concept of the Current flow sounds correct.

The nickel strip is very wide so although its thin there is still a good amount of conductor. Picture rolling that nickel strip into a tight cylinder. It would be substantial cross section wire gauge for the small contribution each cell is adding. It also has a large surface area to dissipate heat and is not covered in a jacket like wire.

If you have a strip welded to 10 cells in a line and you attach the series wire to the end of that strip then you are going to have slightly different load on the cells depending on how close they are to the connection point. Your best bet is to arrange them in a pattern where all cells are as close as possible to the connection point. In a perfect world each parallel pack would be a in circle with the series wire attached in the center. But these issues are not worth obsessing over. Its only an issue if you are drawing a very high C rate and if you skimped on the nickel strip.

Often times you see parallel groups in a line with a wide rectangular nickel plate connecting to the positive end of a group to the negative end of the next P group. In this configuration each cell is the same distance from the next cell so they should share current evenly. In this case you could cut the parallel groups apart and the pack would work pretty much the same way. You wouldn't want to but in theory it wouldn't make much of a difference. The 10 cells would basically be contributing their 5amps each of your 50 total.

So basically you are overthinking it.

Also, 2g wire is absurd for 50 amps or even 100amp bursts unless your pack is 100 feet from your controller. Don't overkill it that much.

Tesla uses tiny wires bonded to each cell that double as fuses when overloaded. If you think nickel strip looks to small you should look at the bonded wires.

 

[DanGT86]

Part of a multipart series on pack building with specific numbers regarding bus bar choices and current carrying capacity of nickel strip.

https://www.electricbike.com/introduction-to-battery-pack-design-and-building-part-3/

 

[DogDipstick]

Someone once discussed this with me. This is what he had come up with. I saved the screenshots, of the equations. This is what you want to know, right? Put numbers where you know they go, and you have the answer, I think. You are competent enough electrical hobbyist to see this. Sorry the last pic was bad resolutions.

Repeat: I did not do this (graphic and calc): I just looked at it and saved it and went over it with the author.