car starts sometimes cranks fine

It sounds like you have an electrical draw. What you need to do is check if there is a draw on the battery with the engine off, and all electrical components off. If there is, remove one fuse at a time until you hit the circuit that the draw is coming from. From that point, you need to check those components.

There are specific direction for checking for an electrical draw on our homepage under the DIY section. Let me know if you have questions or need help.