Low Charge From Alternator
When your car starts to get old, the effect from the alternator will get worse and worse.
The main function of an alternator are to generate electricity for the electrical component of the vehichle and to charge it's battery. The alternator produces this electrical energy by converting mechanical energy.
If you were to try and start your vehicle without an alternator, the battery alone would not be able to produce enough power to get the engine running. Even if you had a new car battery and were lucky enough to turn the engine over, it would probably run for about ten minutes before the power cuts out. The alternator is neccessary to keep the battery recharged and to alternate the electrical current throughout the many component of your vehicle to keep the powered.