When I'm sitting in my car with the key in the accessory position my battery reads 12. However, when I turn the it just click click click clicks.

I slowly charged it three days ago. Now it's dead again.

Should I look to a bad alternator or a bad battery?

 

Yes, a bad battery can still read 12V. A rapid clickclickclickclick when you try to start it may be a bad starter solenoid.

Good news, though. Most big box auto stores can check you battery, alternator, and starter, for free. Of course, you have to take the parts out and bring them in the store first. So, start with the battery, it's the easiest to remove. Then try the starter, because like I said, rapid clicks is probably the solenoid.

 

A weak battery can, and usually will show 12 Volts under no load, but will show much lower voltage under load.

 

Great news, the charging system was fine. The guy that jumped my battery also checked my system and said the battery was reading 110 CCA. He didn't try to sell me a battery so I trusted him.

 

so what happened? the battery charged itself up and is working properly again?

 

The battery wouldn't hold a charge, I got a new one-