Cable size between main & aux batteries?

why get cables that are just adequate? Get cable the same diameter as the battery cables you have ie 4ag. The extra cost is a few bucks/meter over the 8ag. This will also give you the capability of jump starting your primary from your secondary in the event that you flatten your primary.

Leroy

Drew I can give you some cable to do it, its about 35mm2 Cu flexible, thats what I used

Drew, it is hard to answer your question direcly however this may help you understand what is happening to the batteries.
The amount of current that can flow in any circuit is dependant on the voltage applied and the resistance of the circuit. In the case of an alternator trying to charge batteries there is a further complicating factor in determining the max current that will flow which is that the alternator senses the battery voltage and adjusts its output voltage accordingly.

The amount of current that will flow in a circuit can be calculated by the formula

I = E/R Where I = current, E = voltage, R = resistance

In the simple case where the 2 batteries at different voltages are connected together without the motor running the current calculation is:

Say the main batt voltage is 12.4v and the aux battery is 11.2v and the resistance of the cable, clamps, connectors etc. is say 0.06 ohms.
The difference in voltages is 1.2v.

I = 1.2/0.06 = 20 Amps

With the alternator charging a different situation applies. The alternator output is connected to the main battery and it will sense this voltage and regulate the alternator output voltage to what it calculates is required to charge the main battery.
Under some conditions this could be as high as 14 volts in which case the charging current could be at or close to the max alternator output, say 60 - 80 amps. Pushing this sort of current into the battery will soon raise the battery voltage and the regulator in the alternator will sense this and begin to reduce its output current.

In the example above if the circuit resistance is the same and the battery voltages are now 11.2v and 14v the difference in voltage is now 2.8v

I = 2.8/0.06 = 46.66 A

This is the max current that will flow between the batteries under the conditions specified. Clearly the less resistance in the connection between the batteries the better and this means heavier wire. In the example above the voltage drop on the connection cable can also be calculated from the formula by making E the subject.

So E (voltage drop) = I x R = 20 X 0.06 = 1.2v

This would mean the aux battery will never in fact charge fully. This is the problem with many dual battery systems and why batteries are ruined after little use sometimes.
These are just typical values for voltage and resistance and I'm not saying they necessarily apply in your case but you can see the factors that control current flow.

Simple answer is runa as heavier cable as you can fit in and keep the number of joins, connectors etc as few as possible.
Good luck.

Ken