The generic circuit for the charge lamp is +12v ign switched to one side of the panel bulb, alternator small connector to the other side of the bulb. This provides initial field excitation to the alternator and charge indication.
So before start, ign feed = +12v, alternator terminal = 0v. Bulb lights up.
Once running (and charging) ign feed = +14v, alternator terminal = +14v. Bulb goes out.