How to do you read the alternator gauge?

I drained my battery, 3 years old, till it would not start the car. Jumpered and all was fine but ammeter was in the same position. I drove it for 45 minutes and it was still there.

Had battery tested came back good but low on charge put it on barttery charger all night. Next day charge indicator in the center to just right of center.

Shocked me