Calculating a batteries charge rate is a bit complicated, and requires specific technical knowledge about the battery in question.
The alternator is modeled as a constant voltage power supply, lets assume 14V. The battery is modeled as a constant voltage power supply with a resistor in series. The wiring is modeled as a constant resistor. The charge rate of a battery by itself is a function of the voltage at its terminals, the SOC, its internal resistant, and the batteries capacity. The internal Resistance varies widely with battery type, and somewhat with age.
It is easiest to determine charge rates experimentally. Take a charger, ideally one more than 30A that has an amp meter. Discharge the battery to about 50% SOC. Use the resting voltage to determine SOC, which would be about 12.1V. Connect the charger and a volt meter to the battery. After an initial surge, the charge current should level off. Measure the voltage every 10 minutes for an hour. Depending on the charger the voltage may vary. These voltage and current values can be used to estimate charging voltages from your alternator.
If your battery accepts 30A at 50% soc at or near alternator voltage, you can expect about 20A of charging from the alternator, low resistance wiring can get you closer to 30A.
For example, my 510AH bank will accept over 100A at 75% SOC with a terminal voltage of 13.8V. My alternator outputs about 14.1V, and the wiring between the battery and alternator drops about 0.3V at 100A. If that 0.2V drop was removed the batteries would accept at least 150A (assuming the alternator could supply) If you hit the alternators current limit, the output voltage will drop to keep the alternators output under its rating.
As the battery charges the current will taper off. Above 85% it will drop quickly, and it may take several hours to reach 100% depending on alternator voltage.
With low resistance wiring, you can expect peak charging currents of between 10-25% of battery capacity with an alternator voltage between 14.1-14.4V Long wiring, high resistance batteries, or low alternator voltage will reduce this. Charging currents tend to self limit due to the voltage drop in the wiring between the alternator and battery. Obviously there is lots of wiggle room here. At 40% SOC my low-resistance 510AH bank will make out my 200A alternators available power (about 130A) This will taper off to less than 10A at 90% SOC.
Ampacity of a conductor is independent of system voltage. System voltage is only needed to determine insulation type, and most any wire assailable will handle 12v with ease. Wiring for charging a battery (especially from an alternator) should not be sized by ampacity. Instead you should size by desired voltage drop. Ampacity is the minimum wire size, which will result is larger than desirable voltage drops. Ideally I would aim for less than 1% voltage drop (including ground and positive runs) at typical charging current. 20-50A for a smaller battery would be a starting point. 1% would be around 0.1-0.2V. Voltage drop is calculated by using the entire length of charging wiring, the charging voltage, and the amperage estimate, ohms law V=I X R.
