Batterie Problem I think?

[redeye280z]

Hi guys I need some help maybe someone here can help. I think my battery is toast but I want to be sure before I change it.

When the car is running The alternator is putting out 13.8 volts so I know it!s working ok. When the car is not running the battery reads 12.4 volts.

Here is the weird part when the car has been parked for awhile it starts fine but after driving for a little while and the car is hot I have a hard time starting the car it feels like the battery has barely enough power to turn the engine over.

The battery is 3 years old and the car is stored in the winter. I never let the battery die over the winter I do charge it occasionally to make sure it doesn't run dead. Any thoughts would be appreciated.