The main function of a car battery is to start your car and is rechargeable. It uses its charge to power the vehicle systems while it’s parked. Once you start the car, the car generates additional electric power to recharge the battery. Over time, a battery loses its capacity or ability to hold charge. One day, a weak battery will not be able to start your car. When the battery is low on charge, a car will need to be jump started. Regular replacement when the charge level drops can prevent this.