I can help. I assume with battery backup you are talking low voltage like 12, 24, 48, 96 volts?
NEC is not going to help a lot as it does not take operating voltage and voltage drop into account. With lower voltage you have to take voltage drop into account and NEC is not much help. Thus you must design on maximum voltage drop calculated on voltages, maximum current loads, and distance. With that in mind you have to throw tables 310 out the window as it is of no use other than sanity check for short distances.
You design for maximum voltage drop usually 2 to 5% at worse case. That could mean using 1/0 for a 20 amp circuit.
To help you we have to be specific with voltage, current, % of VD, and distances. It is simple Ohm's Law, and a sanity check with NEC.
For batteries cable types dual listing RHW-2, DLO or like works well. You want something with fine strands and flexible in larger cable greater than say #4 AWG and use long barrel terminals with shrink wrap.