In most cases the amp gauge will stay centered, provided everything is working as it should. If the battery is good and the alternator and regulator are functioning correctly, the battery won't be either charging or discharging. The car runs on the battery, the alternator is only there to maintain the battery voltage. The ammeter isn't showing amps output, only charge/discharge. It's kind of a useless gauge IMO. A voltmeter would be a lot more useful. As long as the voltage is good, the amperage is good and the actual amp output of the alternator is irrelevant. The biggest destroyer of an alternator is driving around with a weak battery. The battery will be charging constantly, making the alternator work overtime until it wears itself out. That's why replacement alternators always have a note in the box saying "The alternator is a battery maintainer, NOT a battery charger". Yeah, off topic a bit, sorry.