how do i know if its the altenator or the battery

i can jump start the car with my truck then i unhook it and the neon stays running.. then i shut it off and i cant get it to start back up.. so is it the aletenator or battery

 

have the alt tested at autozone - theyll do it for free.

happened to me - was my battery. a post actually went bad.

 

while the car is running, disconnect the positive battery terminal...
if the car dies, then your alternator is bad

 

Sounds like its the battery to me.

 

X3 for the battery...

 

ok sweet Spud thanks alot!!

i finally had the time to check it out and it was the battery so i filled the little holes with some water and then had to go work.. but it started like it should when i got there so thanks alot.. i wasnt sure if it was the battery cuz i chnage it out (with another old one) but guess it just needed some water which i dont understand how water keeps it charged

 

when the battery get a charge to it, the charge bounces around between all the plates inside the battery....once the water gets a charge, it becomes battery acid....
or sumthing like that