99 Lumina battery keeps dying

[ChevyLum1]

I have a question maybe one of you Chevy guys can help me with. A brief history real quick.
Car is a 1999 with a 3100 v6 in it, bought it in year 2000 with 20k miles on it and has been running fine. Only problem I have had with it was last summer it overheated and I had to replace the thermostat.
Last month my wife drove it to work and when she was going to leave the car wouldn't start. She got a jump and managed to get it home but when we went to start it again a few hours later it was dead. I put it on a deep cycle charger for about 5 hours but it wouldn't charge up. I thought alternator so I took the alternator off to get it tested and tried to recharge the battery again. Took alternator to be tested and autozone guy said amps tested out ok but would need to have it in the car to see if volts were good. I installed alternator back into car and BAM, battery was charged and car started right up. Drove it back to autozone to have battery and alternator checked while in the car running, they checked out fine. Car ran fine until a couple days ago but it wouldn't start again. The same thing happened as I stated above, battery wouldn't charge with alternator hooked up but when I disconected the alternator battery charged up fine and car runs again.
Any guesses as to whats going on here? ALso I noticed that the car has been running kind sluggish and it looks like there is a greasy film building in my engine coolant tank. Think that when it overheated during the summer it messed with the lower intake manifold?

Thanks for any advise, I don't want to buy a new car, I am still making the payments on this one.

[ChevyLum1]

Nevermind about the oily coolent tank, its just gunk from not having my raditor flushed in forever..

[RABarrett]

First, NEVER disconnect an alternator while the engine is running. That can quickly damage a good alternator. If the engine was running while the battery was checked, your results were skewed. Though the likely problem is the battery, having the system checked correctly will likely yield the problem. Ray

[ChevyLum1]

Thanks for the response. I am thinking its the battery, its a Walmart battery and when I had it checked the first time I had just charged it up.

[trannyman52]

if the battery stays charged when disconnected //// the battery should be good/// if this is the case, diodes or rectfier bridge is damaged in
alternator causing battery to drain/// food for thought