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Also, if the centre -zero ammeter read zero, it would mean that the current is not flowing to or from battery. That would mean that the voltage of the alternator and battery are equal. Since the alternator's output should be set to be higher than the battery, it means that the alternator's voltage has dropped to equal that of the battery.

That could be because the alternator has a fault or, it could be that the system demand is beyond the alternator's capacity. This is sometimes the case when some older aircraft have been fitted with a lot of electrical loads that the system was not designed to handle.

If you turned off all electrical loads, the ammeter should show the current flowing to charge the battery.

[color=blue]2)What can you do to check if the gauge/Alternator is working?[/color]

Simply increase the electrical load by turning on the landing light (if possible) and/or pitot heat and/or cabin lights etc. When you do this, one of two thing will happen

If the ammeter swings negative (left of center) you can safely say
1. The ammeter is working and
2. The alternator has a problem as the battery is now powering the aircraft electrical system, either in full or partially - close down all non-essential electrical load … etc

If the ammeter remains on zero (under load), we are back to square one. Do we have a problem with the alternator or ammeter or is all OK. To check, turn of the alternator. One of two things will happen:
1. If the ammeter swings negative, we can conclude that all is OK, the ammeter and alternator are working
2. If the ammeter remains on zero, the ammeter is unserviceable and the alternator is [u]most likely serviceable[/u].

When through with the above checks, don't forget to turning the alternator back on and reduce the electrical load.

I hope I 've got the logic right!
cheers