A good test that will "divide and conquer" your problem between the battery and its connections, or something else:

Turn on the headlights, either under conditions where you can see them or have a helper watch them. Now try to crank the car. If the headlights go just about out, its the battery or a bad cable connection. You can try jumping the car with jumper cables, or removing the cables and cleaning their connecting surfaces. And, as Fisherman comments - it could be your battery ground cable connection at the chassis.

If the headlights stay fairly bright, then you have some kind of issue with the starter or solenoid. The starting system is not drawing current.

At least that's where the probabilities fall for this test.

What test did you use to double check your battery?

Ken300D