replaced the alternator a few weeks ago. Everything was fine until tonight. All the sudden my battery light came on and it acts like it is not charging anymore (running off the battery) Is this a sign of a bad battery? And would it die suddenly like that?
More likely a bad alternator or wiring issue with the alt. You can have a parts store dynamically test the charging system, or take the alternator (and battery if you like) in for bench testing.