keep loosing power!! help!!

my car has been sitting in my garage for a few weeks now. I killed the battery (so i think) while i was installing my new system by leaving the dome light on bla bla. i'm insuring it again tomorrow so i decided to jump start it today. it started fine, left it running for a while but now when i go to start it, it won't start again. The battery keeps loosing power and i don't know why. Also when it's off and i turn on the headlights, or hit the brake pedal or use anything wtih power i'll hear a single "click" coming from my engine. it's an ac delco battery thats only a year old. does anyone know why i keep loosing power and what that click sound is?? any advice would help out..thanks!!

was it doing this before you installed the stereo? the clicking in the engine bay is a relay im sure.

not that i know of. i just bought the car three weeks ago and it died after like two. i got a battery charger on there right now and i'm gonna take it for a longer rip today then hopefully it'll charge up..

Alternator dead?

i don't think it's the alternator..when i boosted it it ran fine for a bit. it's just when i turn it off. I just finished charging it for a few hours..i was out with it tonight and it started fine..guess it was just a really really low battery haha...