car savvy people

[ITH]

got a vehicle question for anyone. my car has a problem, when i drive it its fine but after awhile the battery will loose charge after about 4 or 5 starts and drives the battery will not turn over the engine. I figured it was just the battery not being able to keep a charge because if it was the alternator it would die while i was driving it, cause the voltage gauge does not decrease while im driving it. But i went out after 2 months of not driving it and it starts right up, which leads me to believe the battery can hold a charge, unless it just cant hold a charge while its being used. i dont feel like replacing the battery if thats not the problem so if u guys have any other ideas i would appreciate it.