Originally Posted by 7T1vette

Your ammeter is now working exactly as it should...with a good battery. Anytime a high discharge occurs (after start, electric fan usage at idle condition, etc), the ammeter needle will immediately show the 'spike' of the current surge; then the alternator will respond to recharge the battery to a 'full' condition [needle moves back to zero mark]. If the rate of recovery becomes longer, it indicates that the battery can't be recharged as quickly. However, if the needle returns to the zero mark too quickly, it can indicate that the battery has little ability to take a recharge and needs replacement.

Proper "reading" of the ammeter takes some experience and background knowledge on how it should behave. I'm sure you can find lots of info on the web about how best to utilize an automotive ammeter.