A cars battery will charge a little bit while the vehicle is idling but it will take a long time to fully recharge the battery just by idling the cars engine.
A cars alternator puts out more voltage and charges the battery much quicker when the engine is above idling.
So if you're trying to recharge your battery with the alternator you need to take your car for a drive down the highway to get the battery recharged.
Otherwise put a battery charger on the battery to get it charged back up quickly.