Ok, this has happened to me a few times now.. Once i listened to my radio for like 10 minutes while the car was off.. not enough juice to start the car. The other time i left my lights on for a max of 3 minutes and not enough juice to start it. Sometimes it doesnt feel like it has enough to start it but it still starts. Would this be the battery or the alternator?

 

battery.

alternator charges the battery.
battery starts car to get the alternator to work.

if u killed ur battery by using ur lights/radio with the engine not running, ur batt is prob dead. unless u have a battery charger with a reconditioning feature.