This post is really not one for anyone to diagnose, because there are so many variables.. This is something for the OWNER to determine.... The original poster does not sound mechanical, and should just take it to Nissan. Wishful mechanic people can also do the following.....
Any car owner NEEDS TO OWN a plug in battery charger. You cannot just have the car take care of all the charging. Batteries will always discharge at times, and need to be topped off to 100% charge. Another MUST HAVE is a voltage meter/ tester.
First, the owner needs to establish that the battery is fully charged and strong...
(1) That requires a plug-in battery charger and a "load test".
(2) If the battery passes the load test, then keep the battery and go to step 4.
(3) If load test is bad, get a new, good battery and go to step 2.
(4) With the battery connected, start the car and connect a VOLTAGE METER... Voltage should be around 13.5-14.5 volts. Then try turning on your headlights and check the voltage again. Voltage should still be the same. Then add heat or A/C, voltage should be the same. If voltages check out, go to step 6.
(5) If voltage is lower, then go to Nissan because there is a problem with the charging system.
(6) You have confirmed battery is good, and charging system is good. For a week, on a daily basis, 1) check the battery voltage (with the car off - Voltage should be 12.2-12.6 at rest. 2) with car on, voltage should be in the 14 volt range. If voltages are low at this point, then you have a parasitic draw on your battery when the car is off, or a car charging system problem. Then take it to Nissan.
PLAN B -
If you don't want to be bothered with PLAN A, then stop trying to guess what is wrong and take it to Nissan, who knows the car and its electrical system..