If your car battery is having trouble holding a charge, but you've ruled out the possibility that it is old and needs to be replaced, it could be that the fan belt is damaged or worn. When this happens, the alternator will be unable to charge the battery, so even though jumper cables can get the battery running and start the car, the battery will continue to die until the belt has been replaced or tightened.
The alternator is responsible for keeping the battery fully charged by transferring electricity from the engine. If the belt that powers the alternator wears out, the alternator will be unable to keep the battery charged. In the case of a serpentine belt, other issues may arise when the battery starts to give you trouble.
You can visually inspect a belt for tears, cracks and other signs of damage. The tensile strength of the belt is important as well. A belt that is too loose won't function properly and will need to be tightened.