Alternator output rating
The purpose of a vehicle alternator is to feed various electrical loads as well as to charge the vehicle battery. 

The highest current would be drawn from the battery during cranking, with the alternator making up for that after starting the engine. Likewise, after discharge with the engine stationary. 

Since the battery would get discharged and charged in quick succession, its depletion would be quite low. The charging current would also not be very high during constant-voltage charging by the alternator (no current control).

For the current project, the extent of discharge of the batteries would be a key factor in estimating the maximum charging current that would be required.

Charging current = (Alternator voltage - Battery voltage) / Battery internal resistance

It is assumed that the batteries would be sufficiently depleted after feeding external loads.

It would be necessary to ascertain that the magnitude of the charging current would not be detrimental to the battery. Otherwise, the engine would require to be throttled in order to generate a lower current.

The following additional information may be of use.

An automobile engine generates maximum torque at around 2000 RPM. Best fuel efficiency would be obtained at around that RPM making it the optimum coasting RPM.

Automobile alternators are designed for higher frequencies to achieve size reduction and for ease of filtering out output ripple (with the battery itself functioning as a capacitor at the higher frequencies). 

It has been determined that, above 600Hz, the conversion efficiency of the alternator would drop on account of skin effect and result in flattening of the speed/power curve.

A 12 pole alternator would generate 600 Hz at 6000 RPM. Therein lies the significance of the 6000 RPM rating. 

A 3:1 pulley ratio would ensure an alternator RPM of 6000 at an engine-coasting RPM of 2000.

The IL/IRA VTV rating specifies the maximum output current IL at idling RPM, the maximum output current IR at 6000 RPM at a test voltage of 13.5 V.

The actual alternator output current would be dependant on the load. With the engine idling at 1000 RPM and driving the alternator at 3000 RPM, it would be capable of sourcing 50-80 % of the rated current.

The alternator RPM could exceed 10,000 but should not exceed 18000. 

The flattening of the speed/power curve inherently protects the alternator from overcurrent damage at higher speeds.