Battery wiring question

Wire size in RVs is NO time to penny pinch.

See the WindyNation chart at: https://www.windynation.com/jzv/inf/choosing-right-wire-size

..

Ok there is a point of diminishing returns but the general rule is the bigger the wire the lower the loss.

However.. if you are only drawing say 20 amps.. 2/0 (not 20 but 2/0) is overkill
if you are drawing 200 amps 2/0 is the smallest I'd recommend.

For battery to battery at least the same size as battery to RV.

If you go to the 2nd page of this "12v side of life" is what I used to make up my cables.

The 12volt side of life

Hi,

That may depend on the capacity of the battery management system. If it limits to 1C then there is little point in having thicker wire from the battery to the buss bar.

20? Connected to bus or just to each other?

Connected to each other I would use maybe one size smaller than the largest main cable.

Connected to a bus I would go smaller but large enough to handle max current from each battery.

Yes really.
To each their own.

Ok

18inch each.

This is only to join the positive and negative terminal to create the battrry bank. 4/0 or 2/0 ?

20? Seriously? Are these tiny batteries?

Depends on wire length and peak current drawn.

Google wire gauge and find one of the calculators/tables online.