Disconnect the battery sensor, you can then use what-ever battery you like as the EMU does not know what type of battery is now installed and reverts to a standard charging regime which is OK for both types of battery. I have a Merc and 981 Cayman both have the sensor disconnected, I did it on both cars to get rid of stop/start.
Google found this thread for me - despite last post about a year ago.
An alternator has brushes because it has a slip ring (like a commutator)
The slip ring can corrode and the brushes can stick - both are possible causes of failure after a year standing. Both are pretty rare, and I've only had a failed alternator once after rescuing a lot of "long standing" ...
as a quick check I would start the car and remove battery to check if car can sustain itself on just an alternator also check for voltage change.
It could be that you installed a new battery and while checking for voltage out from alternator you receieve battery voltage
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 ...