Alternator or Battery?

[krzyprk]

How can you tell if it's an alternator or battery problem?

2 nights ago I drove my car and I saw my battery and brake lights on. I parked it for 6 hours, then it wouldnt start. It was low in battery.
I then jumped it, drove...it felt like it was going to stall cuz it was not running right. Made to one block before it stalled on me.

Oh, before I took my car out, it was sitting for like 4 days cuz of the snow we had here in New York.

I took it to a mechanic and told me it's the alternator right away.

Can someone tell me how I can tell if it's the alternator or battery?
I felt like if I replaced the battery, it would have been fine.
Somehow, I think letting my car sit in the snow killed my battery.

[nissandoc]

its the alternator. when it craps out the car will run on battery power until the battery is depleted of its charge. make sure to check the harness to the alternator, b13 sentra's are bad about burning the harness. replace the alternator and charge the battery, if it will take a charge and you should be ok.