Busbar capacity confusion

[spacesuitdiver]

I have a 200A BMS and a couple of 12V 280ah packs I'm interested in paralleling. I am currently trying to determine how to mount the BMS and parallel the packs which involves making some busbars and a tray for the BMS to sit in to hook up B- and another bus bar to parallel the batteries. Worst case load is something like 200A (AC and Induction stove running at full blast) so I figured paralleling is a safe bet and allows me to balance all the cells individually.

Requirements aside I'm ultimately trying to determine the size of the busbar I need and if I should upgrade the provided busbars.

There are a few charts floating around like this one:

Copper Busbar Rating

View Copper Busbar Rating - Approx D.C rating (1). Approx A.C rating. Moment of Inertia. Modulus of Section Z. By Austral Wright Metals.

www.australwright.com.au

And a general formula like this:
1.4/1.6 * mm2 for copper

As an example, given the two methods, the bus bars that shipped with my pack are 2x20mm so 40mm2.
According to the chart 230A (I guess free air means somewhat ventilated)
According to the formula 40mm * 1.4 = 56A

Neither of the two match up which is confusing.

Also, from testing, when using a single pack I can say that the "last" battery in the pack (closest to C-) bus bar seems to be undersized as it seems to get pretty hot and cause some undesired voltage drop on that cell when pulling close to that 200A figure. Stacking two bars solves the problem.

Also also, it's interesting to see that the copper bars coming to and from the BMS are only something like 14mm x 2mm, even smaller than the bus bars!

Thanks for taking the time to read my novel, any help would be appreciated ?

 

[chrisski]

I am putting together two 8S with 280 AH cells and two 100 amp Overkill BMSs. So the busbars for my batteries only need to carry 100 amps, or a little more.

This is how I plan to lay it out:


Part of upgrading your busbars will be if you really want to push 280 amps through both batteries at once, which you probably don't need 560 amps of power. For my system, the check I put in was the class T-Fuse at each battery.

For a 12 volt system, I don't like to see more than 100 amps total, or about 1000 watts for the inverter. For a 24 volt system I don't like to see more than 100 amps total or 2000 inverter watts. When I say 1000 watts for 12 volts, I mean no more than 1000 watts continuously for long amounts of time, but going to 2000 watts for a couple minutes four or five times a day would be OK with really think wiring, but not 2000 watts all day long. The same thing for 24 volts, not more than 2000 watts continuous, but spiking to 3000 watts a couple of minutes at a time two or three times a day would be fine.

I Have used the 12 volt system for a year, and am upgrading to 24 volts.

So IMO, you should consider limiting the system to 200 amps, including busbar, fusing, and wiring, even thought the batteries can push way more. Regardless of what you limit the system to, I don't see you needing to pull 560 amps total, so the busbars need not be that thick.

Also also, it's interesting to see that the copper bars coming to and from the BMS are only something like 14mm x 2mm, even smaller than the bus bars!

I've noticed this also, and am reluctant to dig too much into it. My 100 amp Overkill has two 8 AWG wires going to and from it.