When your engine is idling the alternator is putting out around 80 AMPS and yes the cars battery is charging a little bit at a time but the battery won't get fully charged up until you get the car driving down the highway.
So if you need to charge your cars battery faster you should take the car for a drive down the road at at least 30 to 40 mph but the faster you go the faster the cars battery will charge back up assuming the battery and alternator are good.