Nissan Repair: Electrical, airbag sensor, alternator

Question
I purchased a 94 maxima with 89k about 3 months ago. When I bought it the battery light would come on very very dimly. The owner attributed that to a loose belt. After purchasing the car I placed it in my backyard until I was ready to register it. During which time I would periodically start the car. At one point it lost all power and acted weirdly when I attempted to boost it. After charging the battery I was able to start it but then once again it lost charge. But not completely this time. However at this time the Airbag sensor came on. It has remained on since.



Today I put the car on the road and took it to my mechanic who recommended I replace battery soon. The system was charging 13.2 at idle.  He said the alternator may need changing too.

I also noticed that the radio, which worked fine before, has no power to it at all. I mean that it does not even light up as if it were connected. this has been the case since this morning. I hadn't tried the radio prior to this morning as I had no need for it. It is a detatchable face so I did not re attach the face till today. He said the airbag and radio may be the result of low voltage and that I should first replace the battery.

I replaced it with a diehard gold battery. However the airbag sensor has not got away and the radio does not work either.

Later I saw the battery light come on brighter this time. it would go away while driving but when braking it reappears. Then tonight while stopped at a red light the battery light came on and so did the brake sensor. I believe from prior experience that the brake and battery combo means alternator. however after parking the car and restarting, it came on for a minute or so then went back to only the dim battery sensor coming on when idle.


Please advise me as to what should be done and would like expert advice.


I appologize if my explanation is lengthy but I wanted to provide as much information as possible.

Thanks.

Answer
Danny,

I would change the alternator from what you describe.  Also, check the connections at the battery and be sure all the fuses and links at the positive battery cable junction box are in good order.  I bet the diodes in the alternator have gone out and allowing the battery to backfeed with the engine off thereby draining the battery.