Or it just could be normal.


Lets begin here.
When the engine is not running, power for the lights comes from the battery. The battery has a pretty big reserve of power. If the battery goes to 12.5 Volts when the lights are turned on, its going to stay around 12.5 volts for quite some time. So as long as all the connections from the battery to the lamps are good, everything is going to stay pretty steady. Of course eventually the battery will lose enough power that you will notice the lights dim.



The voltage regulator will attempt to keep system voltage around 14 Volts. At 14 Volts, the lights will draw more current than at 12.5 Volts and they will burn brighter. Some headlights are more sensitive to this than others, but all lights do it.

But at slow idle, the alternator can't produce a lot of power. It should be enough to run the ignition and a little more. Turning on the headlights and stepping on the brake pedal can sometimes max out an alternator's power at 650 or 700 rpm. If in addition to that, the battery is still recharging, and the alternator is already maxed out, system voltage will drop. Then the battery takes over for a little bit and you see it ammeter showing battery discharge, then charge, then discharge. So that's one common scenario and was pretty normal.



Same as the others. Make sure the battery is fully charged using a charger.

Then after starting, let the battery recharge. When the ammeter indicates the battery is nearly recharged (or at least less than 5 amps charging), then try the headlights while holding the engine speed up around 1000 - 1200 rpm. If it still flickers and the ammeter still drops back to discharge, then the alternator is not producing nearly what it should.
First check the belt tension.
Then if that's not is, check for a bad diode or open winding in either the rotor or the stator.