The battery's only job is to start the car. It does not do anything else. The alternator charges your battery while the car is running or you are driving the car. If the car died while you were driving, then you might want to re check the alternator. Now if you jump start the car and you drive it or let it run. for 20 minutes, then turn it off and try to restart the car and it won't start, then your battery is not holding the charge and you will probably need a new battery.