Battery keeps dying(Can't keep a charge)..Got any ideas on what it could be?

[ONYX S-13]

My car battery won't stay charged..I just bought my S13 on the 10th of this month from a friend of mine. We had to jump the car cause he said that his dad left the car door open all night and the interior light killed the battery.So we jumped it , let it sit at idle for like 15mins, to let it charge a lil bit. All of a sudden when I'm about to leave the car dies...So i just bumped started it and it turned on then i drived off to home..The car seemed liked it wanted to dies while i was at stop signs so i would give it a lil gas (So the rpm's would stay at 2k) so it wouldn't die. The next day my friend let me borrow his battery charger so i can see if that worked..I charged the battery for 24hrs.. Put the battery back into my car...It started up first time and lasted about 2 and a half days...The day after that i just went to Kragen and bought a brand new battery and installed it into the car..It worked, but only lasted one day...Now I'm confused. I don't think it has anything to do with the engine cause my friend just rebuilt the engine the day before I got the car..I know i closed the doors hard enough so none of the lights were on.I dunno what else it could be (I don't think it's the altenator(sp?) )..Any ideas on what it could be?

[Startours7]

It's most likley the altrenator. Theres three ways to test it.
Well that I know of anyway.
Let the car run at idel, dissconect the ground off the battery(carefully) and the alt. should be able to keep the car running without the battery.
If you have a volt metter put it to the leads on the alt. It should read13.5-14.5 volts
Take you car to almost any local parts place and they will test it for free.
If it's not the alt. It might be the belt(Most likley not) or a bad ground.
Hope that helped

[old_s13]

If you have a volt metter put it to the leads on the alt. It should read13.5-14.5 volts

The better of the two choices. :)

- Mike

[ONYX S-13]

Yeah I'll just borrow my friends over the weekend and see what's going on..I'll prolly take it someone just to get a professionals opinion as well...Thanks guys.