Wire gauge for low voltage, high current DC application
Forget such things as the motor start current - what matters here is the current required to charge a discharged battery. Charging is low voltage, so insulation is not an issue. In all probability, the existing charging cables are adequately sized (probably only marginally, to save cost) so use wiring which is a bit thicker. For example, if the largest charger cable is 18AWG, use 16AWG for your connections. Start with this in mind, and check the heat developed on charge - a minor rise in temperature is OK, but distinctly warm to the touch means that the cable is undersized. Better to be safe than sorry.