I don't know what you plan to charge with, or the capacity of the batteries that you have, but charging will be based on the capacity of the battery, the depth of discharge and the available charge current. An 8 gauge wire is rated by NEC to 40 amps continuous in a bundle, 6 gauge is rated to 55 amps and 4 gauge is rated to 70 amps. These are pretty much very conservative ratings - some charts rate them to almost double, for use in open air or different types of wire, for example. So two 8 gauge wires have far more than double the capacity of a single 6 gauge, and slightly more than the capacity of one 4 gauge wire. The positive and negative wires need to be the same size, since they carry exactly the same current. This doesn't address voltage drop, which will vary depending on the actual current flowing and the length of the cable. Typically charging from a vehicle alternator is fairly low current, the battery voltage rises quite quickly and this reduced the amount of current that the alternator is regulated to produce. FWIW I have been charging batteries in TC's and trailers with a 10 gauge wire for years with complete success and satisfaction. You just don't want to think you can charge a completely discharged 500 AH battery this way, you can really just maintain the battery.
Brian
