What is the year, model, and engine size of your Nissan? Do you have a DVOM(digital volt/ ohm meter) for testing?
IS the battery light still coming on? Do you have a DVOM(digital voltmeter) in order to test it?
Most likely, there is an alternator problem. For an accurate test you really need a voltmeter.
Do this test, with engine idling remove the positive cable from the battery. Usually if the alternator is bad, the engine will die. If the alternator is good, the engine will still run.
Without a voltmeter there isnt any way to accurately check the alternator's function. If it is a new battery and the alternator is not working, you can get 5-10 days of short (less than 50 miles) drives before the battery dies and wont restart. Make sure you keep all the accessories off unless absolutely needed. Nighttime driving will drain the battery twice as fast with the headlights on.