Low voltage battery systems can be a pain!
You need to consider the system as a whole and work out your voltage drops so that the system still gives acceptable voltage to connected loads when the battery voltage has reached discharge voltage (i.e. the minimum battery voltage).
I select the feeder cables from the batteries/battery charger to the DC DB for very little volt drop (cables end up being much bigger than required for the current). These cables are usually short and are not a problem.
For cables from the DC DB to the individual loads, I use variable voltdrop, and exploit the operating voltage tolerance of connected equipment. For example, if a 48V DC connected load was rated to operate at 48V less 20% (i.e. 38V) then I would select the feeder cable to have a volt drop such that the voltage at the load (with the batteries at discharge voltage) was greater than 38V (allow some safety margin if possible). So the answer is...no fixed volt drop.
If there are any national standards re voltage drops on low voltage (or extra low voltage) systems then these should be followed instead of the method described above.
When calculating volt drop, use DC cable resistance (ohms per meter) and be sure to include the negative return in the length of copper.
I often use 4-core power cables for DC supplies and parallel the conductors in the cable (two positives and two negatives).
Regards
