Lithium cell cable size question - for Paralleling

JaxPilot

This one is for all you smart electrical wizards out there.

I have a question about wire gauge. I am connecting in parallel 3, 4.0V Lithium 40 AH, cells to create a 4.0V, 120 AH battery, which will then be serialized into a 14S3P configuration.

I am using 1/2" wide by 1/8" thick copper for buss bars, which if my math is correct equates to a Cross Sectional Area (CSA) of about 40 mm2. 2 AWG cable has a nominal CSA of 35 mm2. There are a few places inside the case where I cannot connect a straight bus bar, and have to make cables to connect terminals. 2 GA cable, at that small of a distance (6-7") doesn't lend itself well to tight bend radiuses. Also, the connections on the cells are M5 screws, and a 2 GA cable lug with an M5 (#10) stud hole is pretty unwieldy and pricey when you can find them.

All other high current cables are 2 AWG, and the connections coming out of the pack are 2 AWG, and the Serial connections of the cells are all copper bars or 2 AWG cables.

So my question is this. For the parallel part of the cell inter-connections, is 2 AWG still required? It will be for 2 of the 14 batteries. Or does this part of the configuration carry less current than the serial portion, and consequently can have a smaller diameter cable used? Or, is it required (preferred) to use the larger size cable? If smaller is acceptable, would 6 or 8 gauge welding cable suffice?

Thanks in advance!

BGW

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JaxPilot

Here is an example. 4 sets of cells paralleled with bus bars. The two cells at the bottom of the pic have a bar on positive, but need negative connected by a cable. They will join to another cell in the next case. For now I am keeping the original enclosure as we work out the kinks.



This is an 8 gauge welding cable as an example.



This is a crimp test 2 gauge cable but illustrates the challenge.





Sent from my iPhone using Tapatalk

Gregorio

In theory, parallel connections carry a lesser amount of current in relation to the number of connections. Two in parallel would be roughly 1/2, 3 would be 1/3, etc. In practice there are several things that can influence resistance via one path vs another causing imbalance between the otherwise identical parallel circuits.