Custom bus bars for LiFePO4 batteries

It really depends on what the maximum current will be (discharging and charging).

For a 24 volt battery bank, you want less than ~0.1 to 0.2 maximum voltage drop across the bus (i.e., 28.5 volts at the bus inputs for charging, 28.4 or 28.3 volts minimum at the the battery cells.

Cell to cell, voltage variation on the bus, again, less than 0.1 volts drop during charging.

During discharging, we generally use the NEC 3% maximum voltage drop... But to save wasted energy (and excess heat buildup), using heavier bus bars down to 1% drop works well (going heavier for less than 1% drop is just usually a waste of money on copper).

Usually maximum charging current is less than maximum discharge current--So looking at the voltage drop for both, and having different limits is OK. Charging current voltage drop is to ensure cells are equally and accurately charged and where more than a 0.1 volt variation is significant error. And for discharging, just avoiding excess heat and voltage drop to the load (i.e., generally most 24 VDC inverters have a battery cutoff voltage of 21.0 volts--So you look at minimum battery voltage of the battery bank "chemistry" plus the voltage drop of wiring and fuses/breakers).

For example, Lead Acid batteries are generally "dead" (and ruined) at 21.0 volts... And the minimum "operational" voltage under load is (very roughly) around 23 volts. 23 volts FLA battery voltage - 21 volts inverter cutoff = 2 volt drop for bus bars, fuses/breaker, cabling at maximum current.

You should be able to find tables/etc. about copper bus bars... Here is one that talks about copper bus bar vs temperature rise:

https://www.copper.org/applications/electrical/busbar/bus_table3.html

A very detailed design guide here:

https://copperalliance.org.uk/knowledge-base/resource-library/busbars-guidance-design-installation/

Electrical Resistance Calculator:

https://chemandy.com/calculators/rectangular-conductor-resistance-calculator.htm

And the old electrical equation: Voltage = Current * Resistance

-Bill