Discharge

It is actually normal with almost any older system even with a moderate output alternator, and even new systems to some degree. It just shows that the battery is supporting part of the electrical load when the alternator speed is low and its output drops. If you have a new system with a voltmeter, and the voltmeter is showing lower voltage at idle after the battery is well charged up, then the exact same thing is occurring..... you just don't have an ammeter to make it obvious.