started my 08 Altima and put it in gear and it didn't have the power to go anywhere and then the car just died. bought a new battery and everything worked fine. now my battery and brake light on the dash keeps coming on. I tested the alternator last night and it seemed to be charging like it should. what is the problem?
I would get the alternator looked at by a professional. Just searching around on this site, there are a number of reported issues with the alternators on various Altima model years. Some appear to be mechanical failures, where presumably the charging system was ok but the pully failed, sometimes damaging the belt and/or other components.
It's also possible for the charging system to appear to be outputting good voltage at idle, but once there is significant current load on the system the alternator might fail to keep up with the demands of the system. This requires more than just a volt-meter to test.
The initial battery dying could itself be a symptom of a faulty charging system as well. And if that's the case, you would see similar behavior where things seemed "fixed" for a while when the new battery was installed, but as the alternator can't keep the battery charged, you'll again start to see more electrical symptoms over time.