Sounds normal, alternators increase the amperage output as RPM's increase.

If there is more draw/load than the alternator is putting out, then yes it borrows from the reserve capacity of the battery until the output exceeds the load again and then the battery builds the reserve back up. Have seen batteries be totally discharged and cars towed because of idling in severe weather with lights, heater, wipers, electric defrosters and radio turned on. Keep this in mind if sitting in severe weather...do not use what is not needed.

30 amps at idle is excellent output as long as the voltage stays up at the same time.

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