Originally Posted by ConchZ2

At idle, when the alternator is turning slowly, output is reduced and may not be able to keep up with the demands on the charging system. Consequently, the battery has to make up the difference. The reserve capacity of the battery is like a bank account. It stores amps so that the electrical system can withdraw amps when electrical loads exceed the charging system output. Cranking the engine pulls amps out of the battery. So do high electrical loads when the engine is idling. The amps are put back into the battery when the vehicle is driven and the charging system is able to generate more amps than what are needed to keep things running.