Cable rating surely depends on what you are using it for. If you are winching then everything needs to be high rating surely. If you are using it for lights and camping that sort of thing surely theres no need for a second alternator or uprated for that matter.
I'd be making sure theres plenty fuses in there too.
I intanded fitting a lighter duty system for camping and days out to my project truck but she needs to go so no more spending.
One of the biggest problem with under rating the cable is when one battery has a different charge to the other, which can happen very easily and create very large currents passing between the two...
Most cheap split charge relays work on measuring the voltage to decide when to link the second battery into the circuit...
I have a volt meter on my second battery that has an under voltage buzzer, and often if I drive into a petrol station, on a long run, switch off the car and fill her up, then jump back in and start the car, the voltage on the main battery has not fallen enough to disconnect the second battery, and when I start the car I get a beep from the volt meter where the cranking voltage is being drawn from both batteries...
The second problem is when you run a lot of stuff off the second battery with the engine off, then start the car, when the voltage on the main battery rises to the point the relay can switch in, you have a situation where one battery is at a higher voltage than the second battery, and a huge current can flow from the main back into the second...
The third is effectively the opposite of the above, where the car is cranked over a lot running the main battery down, then when the alternator has managed to lift the voltage back up, the second battery is charging the first... again, large currents can be involved...
I used 16mm2 cable all the way on mine, and 200amp relay, with 150amp fuses at both ends of the lead that connects the two batteries.
