Replaced old batteries w/ new. Plugged in charger. Lite on charger is green horn works but when unplugged needle drops all the way back and will not move.

If the green light is on the charger is not kicking on. The light should turn red while charging and then turn green when fully charged. Using a voltmeter set to 20VDC you should see roughly 12.5V-13.V on a fully charged battery. If the voltage on each battery drops below 10V each the charger will not recongize that there are batteries connected. These chargers are designed to search the "system" for 20V, if it doesn't see this it thinks there aren't any batteries in the scooter. In addition even if the batteries show good voltage it doesn't mean there good. You have to "load test" the batteries with a load tester to simulate the scooter driving.

The rechargeable batteries are no good or a connection in the battery holder is loose.