a weird thing happened today. i couldn't start engine, but i took the car for a ride and drove upto 50 miles. Thankfully i came back home. using a jumper cable, after prolonged efforts the engine started, but soon after it died. i understand this is a battery problem. but why would it die, once it kicked off. doesn't the alternator takes care of the sparking after it kicks off? pls give me some insight. thanks