Battery dies after 30 mins?

[slimsol]

I just bought a 1999 explorer sports with 99k miles on it. According to the previous owner.It was sitting for a while. I have been driving it for couple of days. Last night i was cleaning the car and left door open for aout 30 mins. ( dome lights and mirror lights were on). And my car battery died.
1. Do i have to check battery fluid level?
2. New battery?

 

[chris2010]

my girlfriend has one and it did the same thing so we thought it might be the altenater so we found a redneck way to check it take the battery off the car and charge it once fully charged put it back on the truck and start it if the altinater is bad it wont run for to long.

 

[SoNic67]

Charge the battery with a charger. Start the truck and measure the voltage - 14.4V is OK, 13 or less is bad alternator.
Do NOT try to jump-start the Explorer with a depleted battery. It will fry the alternator!

 

[slimsol]

Ok so i removed the battery cable while car was running and it was fine. Does that mean my alternator is fine? Should i just replace the battery? Does battery become useless if car was sitting for few months

PS i dont have charger to charge the battery.